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

Hasnaa Chennaoui Aoudjehane, Houda El Kerni, Wolf Uwe Reimold, David Baratoux, Christian Koeberl, Sylvain Bouley, Mohamed Aoudjehane

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## ► To cite this version:

Hasnaa Chennaoui Aoudjehane, Houda El Kerni, Wolf Uwe Reimold, David Baratoux, Christian Koeberl, et al.. The Agoudal (High Atlas Mountains, Morocco) shatter cone conundrum: A recent meteorite fall onto the remnant of an impact site. Meteoritics and Planetary Science, 2016, 51 (8), pp.1497-1518. 10.1111/maps.12661. hal-02320270

HAL Id: hal-02320270

https://hal.science/hal-02320270v1

Submitted on 22 Aug 2025

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

Hasnaa CHENNAOUI AOUDJEHANE<fnref n="1" />*, Houda EL KERNI<fnref n="1" />, Wolf Uwe REIMOLD<fnref n="2" />,<fnref n="3" />,
David BARATOUX<fnref n="4" />,<fnref n="5" />, Christian KOEBERL<fnref n="6" />,<fnref n="7" />, Sylvain BOULEY<fnref n="8" />, and Mohamed AOUDJEHANE<fnref n="9" />

*(Received 30 September 2015; revision accepted 20 March 2016)

**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 km<fnref n="2" />) 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⁻<fnref n="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

<footnote n="1">¹Hassan II University Casablanca, Faculty of Sciences Ain Chock, GAIA Laboratory, BP 5366 Maârif,
20000 Casablanca, Morocco</footnote>

<footnote n="2">²Museum für Naturkunde Berlin—Leibniz Institute for Evolution and Biodiversity Science, Invalidenstrasse 43,
10115 Berlin, Germany</footnote>

<footnote n="3">³Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany</footnote>

<footnote n="4">⁴Géosciences-Environnement-Toulouse, Université Paul Sabatier CNRS & IRD UMR 5563, 14 Avenue Edouard Belin,
31 400 Toulouse, France</footnote>

<footnote n="5">⁵Institut Fondamental d'Afrique Noire Cheikh Anta Diop, Dakar, Senegal</footnote>

<footnote n="6">⁶Natural History Museum, Burgring 7, 1010 Vienna, Austria</footnote>

<footnote n="7">⁷Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria</footnote>

<footnote n="8">⁸GEOPS—Géosciences Paris Sud—Université Paris Sud—Bât, 509—91405, Orsay Cedex, France</footnote>

<footnote n="9">⁹302 Boulevard Panoramique, 20150 Casablanca, Morocco</footnote>

<footnote>*Corresponding author. E-mail: Chennaoui.hasnaa@gmail.com</footnote>
the observed shatter cones belong to an ancient impact structure that has been almost entirely eroded.

INTRODUCTION

Collisions with asteroids and comets have affected the Earth since its formation. The most ubiquitous traces of this collision history are impact structures. To date, 189 terrestrial impact structures have been confirmed and constitute a geological record that spans ~2 Ga (source: Earth Impact Database at www.passc.net/EarthImpactDatabase/, last consulted in February 2016, and includes the recent discovery of the Saqqar impact structure in Saudi Arabia by Kenkmann et al. 2015). The size range of known impact structures extends from tens of meters to some 250 km.

Traces of impact events are not equally preserved. Exposed impact structures appear as circular features in aerial and satellite imagery, or form circular or subcircular depressions, sometimes surrounded by rims or annular rings, also seen in digital elevation models. Only 128 impact structures are exposed on the Earth's surface (Hergarten and Kenkmann 2015). Others are completely covered by postimpact sedimentary strata and have been detected from exploration drilling, or through initial geophysical studies followed by drilling. The topographic expression of an impact structure can be very subtle or even entirely absent for old and largely eroded structures. Also, impact evidence is not systematically associated with a preserved impact structure. For instance, spherule layers in Archean and Proterozoic strata of South Africa and Australia represent evidence of large impact events, for which impact structures seem to have been erased from the Earth's surface (e.g., Glass and Simonson 2012; Reimold and Koeberl 2014).

In Africa, only 19 impact structures have been recognized to date. In North Africa and on the Arabian Peninsula, 13 impact structures have been discovered so far (Youbi et al. 2011; Reimold and Koeberl 2014; Kenkmann et al. 2015). A systematic search program for impact structures has been conducted in Morocco

since 2010, using the satellite imagery available through the Google Earth software, to identify and select circular features to be visited for ground study and sampling to confirm or disprove the possibility of an impact origin (Chaabout et al. 2015). Twenty-eight possible structures were identified in the territory of Morocco, of which 19 had to be discarded because alternative geological origins had already been established, and the remaining 9 were visited. None of these has yet yielded positive evidence for an impact origin (Chaabout et al. 2015).

In most impact events stony, iron, or icy impacting bolides are nearly entirely vaporized directly upon contact with the target, due to the shock pressure levels associated with high-speed collisions (Melosh 1989). Rare meteorite fragments have survived formation of a large impact structure in only a few cases (e.g., Maier et al. 2009). Such fragments were either the products of spallation of the projectile, or were large enough to survive atmospheric entry and small enough to be decelerated by atmospheric drag. They usually show signs of intense deformation. In the case of iron meteorites, the so-called shrapnel form (e.g., at Kamil, Henbury, Sikhote Alin, and Canyon Diablo) is prominent. On the other hand, meteorite finds (or falls) are rarely associated with impact craters, generally because the terminal velocity of small fragments is too low to cause an impact crater (Melosh 1989), the only exceptions being the Kamil (Egypt; Folco et al. 2011) and Carancas (Peru; Kenkmann et al. 2009) craters.

Iron meteorites have recently been found by local people, and reported by meteorite dealers, near the village of Agoudal, Morocco, close to the city of Imilchil in the Central High Atlas mountain range (Fig. 1). Our team conducted field work in this region in 2013 and 2014 in order to obtain meteorite samples, to verify whether all recovered pieces are from the same fall, to outline the strewn field, and to gain an idea of the total mass of the meteorite. We submitted the

Fig. 1. a) Schematic geology of the High Atlas region with localities of Agoudal village, the shatter cone site, and the Akhiam valley in the High Atlas Mountains of Morocco, as well as the anticlinal ridges with Jurassic intrusions. b) RGB color composite satellite image overlain on a shaded relief map of the Agoudal area. The color composite image corresponds to the 2nd, 3rd, and 4th main axes of a principal component analysis (PCA) applied to a LANDSAT 7 image of the Agoudal area (scene LE72000382001246EDC00). The first axis corresponds to the brightness of the scene and is sensitive to shadow and illumination condition and is not represented here; this choice emphasizes lithological differences. The shaded relief map was calculated using SRTM (Shuttle Radar Topography Mission) data with a resolution of 30 m pixel⁻¹. The data sets have been projected into the Universal Transverse Mercator (UTM) system.
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coordinates of the find and its classification as an IIAB iron meteorite to the Nomenclature Committee of the Meteoritical Society. This submission was accepted in April 2013; the meteorite was named "Agoudal" (Chennaoui Aoudjehane et al. 2013; Ruzicka et al. 2015). Following the publication of the fall coordinates, several groups of Moroccan and international meteorite hunters, as well as some scientists, visited this region to collect fragments of this meteorite. The estimation of the total weight was more than 100 kg, but since the cataloging of the Agoudal meteorite, there have been several more large finds (including fragments of 190 kg, 92 kg, 74 kg, or 52 kg). An estimated 2 tons of meteorite fragments have been collected at the time of writing, according to the information collected from different local sources about material sold.

Among the international visitors to Agoudal was a group of Russian dealers accompanied by researchers from the Vernadsky Institute, Moscow, who noted the presence of shatter cones and a kind of breccia in an outcrop (N31°59′12.7", W5°30′57.5") of marly limestone located within the area where meteorite fragments had been recovered, ~5 km to the southwest of the village of Agoudal (Sadilenko et al. 2013; Lorenz et al. 2015). Following these discoveries, some writings in Moroccan newspapers suggested that two subcircular lakes termed Isli and Tislit, located some 25 km to the northwest of Agoudal village, could be impact craters. We conducted detailed field investigation around these lakes and could not find any evidence in support of this hypothesis. Thus, the impact idea for these two lakes was rejected in Chaabout et al. (2013), a conclusion that was fully supported by Ibouh et al. (2014), who argued for a tectonic origin of the lakes.

The occurrence of shatter cones and meteorites within a small area southwest of Agoudal is unusual and has led some authors to propose the possibility of a genetic relationship between the meteorite fall and the formation of shatter cones (Rochette et al. 2014; Lorenz et al. 2015), but these views were promptly disputed (e.g., El Kerni et al. 2014). Three visits to the Agoudal site were conducted by us in May and October 2013, and in May 2014. Here, we present new remote sensing and field observations including first structural data, an extended occurrence of shatter cone sites, and the evaluation of the distribution of meteorite fragments. The data are examined for any hint of a circular structure or feature that could represent an impact structure or remnant thereof, and to place constraints on the original size and age of a putative impact structure, with respect to the age of the Agoudal iron meteorite fall. We conclude that the Agoudal meteorite fall postdates the formation of shatter cones. In light of these observations, we finally discuss the likelihood of a

coincidental meteorite fall on a previously formed impact structure.

## GEOLOGY OF THE AREA

### Overview of the Geology of the Atlas Mountains Belt

The High Atlas is the highest mountain belt of Morocco, reaching more than 4000 m in elevation (Mt. Toubkal, 4165 m). The High Atlas Belt extends for ~2000 km in a SW-NE direction from the Atlantic Ocean to Algeria (Fig. 1). The width of the mountain belt varies between ~50 and 100 km and is limited by the Meseta domain to the north and by the northern boundary of the West African Craton (Anti-Atlas Belt) to the south (Milési et al. 2004). The Atlas System (High Atlas and Middle Atlas) was formed through inversion of Triassic–Liassic rift basins during the Africa–Eurasia convergence (Frizon de Lamotte et al. 2008).

A simplified geological map of the High Atlas Belt with the location of the Agoudal area is presented in Fig. 1. The basement to the High Atlas fold and thrust belt comprises Precambrian to Cenozoic rocks. Precambrian metamorphic rocks and granites are topped by late Precambrian volcanics in the Ouzellagh Block (Western High Atlas). The Precambrian basement is overlain by mostly clastic Paleozoic and Mesozoic–Cenozoic successions. The pre-Mesozoic basement can be observed in several inliers of the High Atlas, forming the most elevated areas of the Western High Atlas. The Mesozoic successions are mostly of Jurassic age; they crop out almost exclusively in the Central High Atlas and in the Atlantic basin south of the city of Essaouira. The Cenozoic deposits are essentially absent along the belt axis but characterize the High Atlas boundary against the Meseta plains where Neogene formations were deposited. The Late Visean-Early Westphalian tectonic event (380–280 Ma; Main Variscan Phase) produced tight folds associated with metamorphism and granite intrusions. Folds show NE-SW to N-S axes and have subvertical and generally E-dipping axial planes with western vergence, with pervasive axial plane cleavage. The *P*–*T* conditions of metamorphism did not exceed the low-grade greenschist facies during the main Variscan folding, except in the regions close to granite intrusions. Overall, the Variscan structural pattern is dominated by major ENE-WSW to NE-SW dextral fault zones (especially in the Western High Atlas). The Imilchil anticlinal ridges (Fig. 1) were developed as “salt walls” (Michard et al. 2011) during the Liassic-Middle Jurassic interval above the dominant NE-trending basement faults. Magmatic intrusions played also an important role in the genesis of the Central High Atlas
<figure><img src="image_5.png" /><figcaption>Fig. 2. Photos of (a) a synsedimentary fault and (b) slumping in the Bajocian limestone on the Msadrid-AAK anticlinal ridge, close to Imilchil on the road to Agoudal, position N32°10′01″/W5°35′54″.</figcaption></figure>

ridges (Ettaki et al. 2007) by accentuating the relief. The largest intrusions were emplaced as sills into the sedimentary supracrustals, particularly beneath competent Liassic carbonates.

## Geology of the Agoudal Study Area

The village of Agoudal is located ~20 km to the southeast of the town of Imilchil, and ~25 km southeast of lakes Isli and Tislit (Fig. 1). Small folds with amplitudes ranging from several meters to tens of meters can be observed around Agoudal. Disharmonic folds or slumping features and, in other parts, thickness reduction in carbonate beds as well as changes in lithology are contemporary with the formation of the anticlinal ridges (Fig. 2). Salt tectonics could explain the early development of the ridges during Liassic-Middle Jurassic times (Bouchouata et al. 1995; Ettaki et al. 2007; Michard et al. 2011). However, the fault system within pre-existing diapirs accommodated magmatic intrusions and played an important role in the formation of the Central High Atlas ridges (Ettaki et al. 2007).

The stratigraphic series exposed at Agoudal (Fig. 3) is related to the Bin El Ouidane Marly Limestone Formation that was deposited during the Bajocian period (Middle Dogger, 170 Ma) (Milhi 1997).

The location of the study site is at the northeastern termination of the Ikerzi ridge (Fig. 1). Calcareous deposits including blue marl with brachiopods and oolitic limestones are exposed in the Akhiam valley (see Fig. 1). They are overlain by a series of blue marl with reef limestones exposed at the top of the hills. Beyond the anticlinal ridge to the southeast and northwest, varicolored marls, sandstones, and limestones with oolites and oncolites of upper Bajocian age are observed. We have observed up to 20 cm large

ammonites (Amonites Discites sp.; Milhi 1997) some 2 km from the study area that confirm the Bajocian age of this formation. The Atlas structure is visible in the Akhiam valley; it is characterized by a set of narrow anticlinal and wide synclinal forms with isoclinal flanks of shallow dipping strata. The strata are oriented subhorizontally toward the center of the syncline in the central parts of these structures, above the “calcaire-corniche” (calcrete) of the surrounding peaks. Faults and folds forming anticlinal ridges orientated N60, or in some cases N120 or N-S, are observed at the Agoudal shatter cone sites (Fig. 4); they can be directly related to the Central High Atlas shortening. The anticlinal fold systems are associated with reverse faults that have been active during the Tertiary inversion (Frizon de Lamotte et al. 2008; Michard et al. 2011).

## FIELD OBSERVATIONS

### Description of Outcrops

Sadilenko et al. (2013) and Lorenz et al. (2015) identified and described shatter cones at a single outcrop located 5 km southwest of Agoudal village (at N31°59′12.7", W5°30′57.5"), in a bluish—when weathered beige—formation of marly limestone with oolitic limestones (Fig. 3, SC1). The shatter cones there crop out on the midslope of a hill in half to several meter wide exposures of marly limestone, with intercalated strata of mostly fragmented calcareous material, in the form of discontinuous blocks without apparent structure, in comparison with the regular stratification in the surroundings of the site (Fig. 5). The slope is partially covered by talus and the breccia described below.

We found a second outcrop with shatter cones at N31°59′17.3″/W5°30′53.6″ (SC5-6-7-8), 150 m to the
<figure><img src="image_6.png" /><figcaption>Fig. 3. Stratigraphic column of the Mesozoic-Cenozoic series of the Central High Atlas, after Frizon de Lamotte et al. (2008) and Charrière et al. (2009), modified by Michard et al. (2011) and by the authors.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 4. Example of structures observed at Agoudal that correspond to the regional structural pattern of the Central High Atlas showing the N60 to EW folding around the shatter cone locations. Width of the area shown is ~2 km.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 5. General view of the Agoudal site showing the locations of all in situ shatter cone occurrences observed in this study (7 locations with one or more outcrops totalling 11 outcrops), as well as the locations of outcrops exposing fine-grained, poorly sorted breccia. The outcrop discovered by Sadilenko et al. (2013) is located on the central hill (SC1). This central hill is without apparent stratification compared to the surrounding exposures that show regular subhorizontal attitude of the beds. The width of the area shown is ~1.5 km.</figcaption></figure>

northeast of the main (discovery) outcrop, on the other side of a broad Oued interpreted as the trace of a major fault (Fig. 6, SC6). A third, several meter wide outcrop (SC10bis; note, SC10 represents an observation of SC float nearby) with shatter cones was discovered on the plateau to the southwest of the “discovery hill,” at N31°59′04.9″/W5°31′02.0″. Between SC1 and SC6, the unconsolidated erosion debris on the hill-top contains numerous centimeter- to (rarely) decimeter-sized fragments of shatter cones. Shatter cones were also found

in a pit where a large piece of the Agoudal meteorite had been excavated, ~250 m west of SC1 (Fig. 7, SC4). Three other in situ shatter cone-bearing places were also discovered (Fig. 5, SC11, SC12-13, SC15). All shatter cones occur in the ammonite- and brachiopod-bearing formation belonging to the Bin El Ouidane Formation of lower Bajocian age. At sites SC2-3-9-10-14, no in situ shatter cones but only float was observed.

On the north slope of the hill where shatter cones were first discovered, and below the main shatter cone
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Fig. 6. Photo of a typical outcrop with well exposed shatter cones (N31°59′17.3″/W5°30′53.6″, SC6). This outcrop is situated 150 m to the northeast of the first shatter cone occurrence (SC1) discovered by Sadilenko et al. (2013). Arrows emphasize some well-formed shatter cones.

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Fig. 7. Shatter cones found in an excavation pit where a large Agoudal meteorite piece was recovered (SC4).

outcrop (see Fig. 5), a calcareous breccia was excavated to a depth of 25 cm over a few square meters. It is a poorly sorted fine-grained breccia of marly limestone (see Figs. 5 and 8, Breccia 4) that may contain reworked marl fragments carrying shatter cone segments (see Figs. 5 and 9, Breccia 1–3). This breccia likely corresponds to the one described by Lorenz et al. (2015)—at least the images presented by these authors strongly suggest this. The presence of shatter-cone marl fragments within the breccia indicates that the breccia was deposited after shatter cone formation. A very similar, poorly sorted breccia was observed close to the SC10bis shatter cone outcrop at N31°59′04.9″/W5°31′02.0″ (Fig. 5, Breccia 2). We note that all breccia occurrences (Figs. 8 and 9) are similar with respect to their clast appearance, but differ in terms of presence of shatter cone clasts or in terms of the mean clast sizes and sorting.

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Fig. 8. Fine-grained breccia on a marly limestone outcrop on the northern flank of the central hill with several exposures of shatter cones (see Fig. 5, Breccia 4).

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Fig. 9. The poorly sorted breccia at the SC1 shatter cone discovery site containing reworked marl and limestone, as well as fragments of shatter cones (see Fig. 5, Breccia 1–3 and Breccia 2).

In total, 11 shatter cone outcrops of limited extent (at seven different locations), many occurrences of shatter cone float, and four breccia occurrences were found (Fig. 5). Localities of shatter cones, breccia, and Agoudal meteorite finds are summarized in Fig. 10.

## Characteristics and Distribution of Shatter Cones

In the Agoudal area, floats of small, mostly < 4 cm size (Fig. 11) shatter cone fragments occur scattered over an area of ~900 × 400 m extent, but in situ shatter cones are restricted to a region of 500 × 300 m straddling the eastern boundary of the meteorite strewn field (Fig. 10a). Shatter cones occur in Middle Jurassic limestone and marl (Upper Bajocian) of ~168.5 Ma age (Gradstein et al. 2012). The local limestone belongs to the NJT 10b nannofossil zone, which corresponds to the Parkinsonia parkinsoni ammonite zone (Lukenender,
<figure><img src="image_13.png" /><figcaption>Fig. 10. a) Satellite image showing the locations of visited outcrops and the location of shatter cone finds. The faults and dip directions of sedimentary layers around the Agoudal study site are also reported, as well as the geographic distribution of Agoudal meteorite fragments (strewn field). The direction of the fall is SE to NW (dashed arrow). Thousands of very small pieces were recovered on essentially three plateaus, buried under a few centimeters of soil (yellow squares). The satellite image used to report geological measurements is a WordView 2 scene, covering 50 km², with a panchromatic channel at 0.5 m and 2 m resolution for multi-spectral channels, which was acquired on 30 November 2014. b) Shatter cone occurrences (in situ or as float) and the fault locations in the study area. NE/SW trending regional Atlasic deformation (jointing) is not reported. c) Bedding orientations measured at and around the study area.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 11. Examples of assemblages of several centimeter-sized shatter cones from Agoudal, with apices pointing in varied directions. a) Cluster of shatter cones in horse tailing fashion. Note that many cone segments seemingly originate from fractures (compare Hasch et al. 2016). b) Concave and convex developments of shatter cones, in this specimen illustrating the pervasive character of this fracturing phenomenon. c) Bluish marl in calcrete breccia, with apices pointing in distinctly different directions. d) A rare specimen with two shatter cones pointing into opposing directions, away from a fracture forming the base of both cone features.</figcaption></figure>

personal communication). Shatter cone-bearing outcrops occur within a largely gravel-covered terrain as, generally, < 1 m² occurrences. Shatter cones are well developed: curved surfaces of various orientations and with diverging striations occur in the rock offering unequivocal evidence for hypervelocity impact deformation (Baratoux and Reimold 2016). No meteorites were found in the main shatter cone area. Larger meteorite pieces were collected at least 100 m from the shatter cone discovery site (Fig. 10a).

## Breccia

A poorly sorted breccia was observed exclusively at or just below the surface within the shatter cone area (e.g., Lorenz et al. 2015). Our breccia samples (Fig. 9) can be directly compared with the sample shown by Lorenz et al. (2015) in their fig. 5b. The authors reported the occurrence of small fragments of the Agoudal meteorite in this breccia. They also claimed to have observed a remnant of melted material. From our observations, this material is composed of fragments of, in a few cases shatter coned, carbonaceous marl/shale with slightly magnesium-carrying calcite and relatively low but somewhat variable

amounts of quartz microclasts, some potassic phyllosilicates (montmorillonite, illite), and accessory iron-oxides.

There are, however, two populations of carbonate material: cement between the marl/shale fragments and carbonate clasts, some of which show striated fractures, that is, they are fragments of shatter cones. The clast population does not show any notable deformation and no evidence for shock deformation. Cathodoluminescence analysis of several Agoudal breccia samples was carried out with a Lumic HC3-LM CL-microscope, equipped with a DX20HC camera system, in the Museum für Naturkunde Berlin. Image acquisition was performed with a 14 kV voltage at currents between 0.1 and 0.3 mA. Cathodoluminescence of the carbonaceous shale fragments of several millimeter to several centimeter size is characterized by a uniform orange luminescence as is typical for calcite (Fig. 12). Intensity variations of the orange luminescence result from variable concentrations of carbonate in the montmorillonitic/illitic phyllosilicate groundmass. A few specks of quartz show bluish luminescence, and traces of feldspar and apatite grains luminesce greenish. Particle sizes of the marl/shale are
<figure><img src="image_15.png" /><figcaption>Fig. 12. Microphotograph of a thin section of the Agoudal breccia in (a) crossed polarizers and (b) cathodoluminescence. The width of field of view is 3.5 mm. The area shows several angular shale clasts (C) set into a groundmass (G). The CL image clearly shows the typical orange color, as both phases contain considerable carbonate; both carbonate generations have slightly different CL appearance.</figcaption></figure>

submicroscopic (i.e., <20 µm). Microfossil particles show the same orange luminescence as the marl groundmass. Locally, the carbonate groundmass of the breccia that cements the marl/shale fragments has a slightly different, milky-lavender-orange luminescence.

This breccia is a result of erosion, weathering, and cementation processes. Considering the absence of any evidence of phases (such as impact melt) or deformation, especially shock metamorphic effects, of clasts, we consider it unlikely that this breccia could be an impact breccia. At some places, it contains fragments of shatter cones (Fig. 5). From these combined observations, and from the fact that we have observed breccia exclusively on superficial material, with no

indication of existence of deeper occurrences (e.g., occurrences of breccias on tilted layers exposed at the surface), it is concluded that it represents a recent secondary deposit akin to calcrete, whose formation clearly postdates shatter cone formation. Therefore, the presence of meteorite fragments noted by Lorenz et al. (2015) within this breccia does not necessarily imply a genetic link between the breccia and the meteorite fall, but meteorite fragments may have been incorporated and cemented in the superficial recent deposit.

## Structural Description and Mapping of the Visited Area

A structural survey was conducted over an area of ~1000 m × 700 m, surrounding and including the main shatter cone outcrops of the central hill and all other outcrops with shatter cones. We have taken orientation measurements for joints, fractures, and faults, as well as dip and strike directions of the strata. A WordView 2 satellite image covering 50 km² with a panchromatic channel at 0.5 m, and 2 m resolution for multispectral channels, acquired on 30 November 2014, was used as a background to display our data and field observations with ArcGis software (Figs. 10a–c).

## Strike and Dip of Sedimentary Layers

The carbonate formations of the Agoudal area form a wide anticline at the northeastern termination of the Ikerzi Ridge (Fig. 1). The area with occurrence of shatter cones is limited to the northeast by a N150-N180 trending structure. Steeply dipping faults on the north and south sides of the shatter cone site show displacements, never exceeding a few meters, in the Jurassic limestone formations (Figs. 13 and 14). Here, dark gray shale of marl to marly limestone composition hosts shatter cones (the second outcrop at N31°59′17.3″/W5°30′53.6″) in a layer that crops out a few meters below a limestone with fossils. The beds at this location are oriented vertically and, slightly, overturned to the north (Figs. 13 and 14). These vertical layers show a cleavage fracture perpendicular to the stratification (Fig. 15). Two other vertical limestone layers are reported at the top of the central hill, one showing a WNW-ESE bedding orientation and the other a NE-SW orientation (Fig. 10c).

Measured strike and dip values of limestone layers in the surveyed area (Fig. 10c) are mainly in concordance with the regional geology of the Agoudal area (Figs. 4 and 5), with subhorizontal, up to 20° dips and a variety of strike orientations, except to the northeast of the main shatter cone outcrop (central hill) where a vertical to overturned layer trending N150-N160 was observed along the edge of the shatter
<figure><img src="image_16.png" /><figcaption>Fig. 13. Fossiliferous limestone of Bajocian age from the northeastern part of the main shatter cone occurrence at the central hill. Note the change from vertical attitude to slight overturning. See text for detail. Backpack for scale ~50 cm high.</figcaption></figure>

cone outcrop area. On the main shatter cone discovery hill, we did not see any structural features or stratification due to the apparent chaotic mixture of limestone, marl, and red soil—with locally tabular or irregular breccia deposits.

## Faults and Fractures

Several fracture systems at N70 to N110 orientations were observed in the fossiliferous limestone beds. The measured orientations are reported in Fig. 10b, and on a rose diagram to represent the distribution of azimuths of faults and fractures (Fig. 16). The main orientations correspond to the regional High Atlas fracture systems and are related to the evolution of the main deformation due to the Atlas shortening of Alpine or Miocene age (Frizon de Lamotte et al. 2008; Michard et al. 2011). These fractures have not been reported on the structural map

(Fig. 10b) in order to focus on local systems of fractures that might be related to the shatter cone formation event. The rose diagram shows two major joint orientations: one at N170 to N180, which is accompanied by other subsidiary fracture orientations— in addition to the N60 Atlantic fault trend (Fig. 16). The rose diagram allowed choosing the fault orientation related to the Atlantic fault shortening, in order to not report them on the structural map (Fig. 10b). The plotted fractures appear to be restricted to the shatter- coned area; they are not related to the regional High Atlas fracture system. Meter-scale displacements were observed in the northeastern part of the study area along 100 m long fault sections (Figs. 13 and 14). In the central and southern parts of the study area, the orientations of fractures are much more scattered (Fig. 10b). The vertically oriented, N150-N160 trending layers of fossiliferous limestone along the northeastern limitation of the shatter cone-bearing area are separated by dextral faults with displacements of a few meters (Figs. 13 and 14). These vertical layers also show a cleavage perpendicular to the stratification (Fig. 15).

## Agoudal Meteorite Fragment Distribution and Age

Agoudal is a IIAB iron meteorite (Chennaoui Aoudjehane et al. 2013). During a first field mission in February 2013 we collected coordinates of most pieces recorded in the area; further data were added during the following field missions with new finds and new collection areas. The total estimated mass is up to 2 tons of material recovered. All meteorite fragments were buried by a few centimeters (all small pieces) to ≤2 m (a few large pieces occurred as deep as this) below the surface—all within alluvium. The recovered meteorites are weathered and show two distinct morphologies: irregularly shaped shrapnel pieces of centimeter size, and some smaller, rounded, bullet-shaped fragments (Fig. 17). There are three “plateau” areas with thousands of small pieces (1–100 g) separated by areas where finds are rare (Fig. 10a). Many meteorites in the 100–1000 g range and a few big pieces, especially in the northwestern collection area, were reported. This distribution delineates a strewn field of ~10 × 4 km elongated in SE/NW direction (Fig. 10a). The strewn field drawn by Lorenz et al. (2015) did not take into account the new recovery location to the east and findings of a few large pieces in the northwest. These new discoveries make the strewn field a bit longer and change the direction of the fall by a few degrees toward the southwest (Fig. 10a).

Cosmogenic radionuclide abundances in the iron meteorite were measured at ASTER (CEREGE, Aix en Provence, France) and indicate a pre-atmospheric
<figure><img src="image_17.png" /><figcaption>Fig. 14. Normal and overturned beds separated by faults in the northeastern part of the main shatter cone occurrence at the central hill. Width of the area shown is around 500 m.</figcaption></figure>

impactor of a few meters in size, and a late Pleistocene age for the fall of $105 \pm 40$ kyr (Hutzler et al. 2014; Hutzler 2015).

# DISCUSSION

## Discussion of Geological and Structural Observations

No circular structure (or part thereof) is in evidence from remote sensing or structural geological observations around the shatter cone locations, or even farther away from the shatter cone area. The map of dip variation in the study area shows clearly horizontal to subhorizontal beds occurring within the limits of the prospected area (Figs. 5–10). However, the shatter cone area is limited to the northeast by vertical to overturned layers (Figs. 10, 13, and 14). Only two vertically oriented layers were observed in the central part of the hill with shatter cones.

On and around the shatter cone area, we can report numerous centimeter-sized fractures and some localized small faults. Most faults and fractures plotted in the rose diagram (Fig. 16) are related to the main Atlas deformation. The fine fracturing noticed in the vertical layers (Fig. 15) seems to be local and not related to the major Atlas-age folding in this area. The vertically oriented and overturned layers are separated by small, radially oriented faults; otherwise, the fracturing seems to be totally chaotic, without specific

preferred direction, especially in the central hill (Fig. 10b).

The fact that no circular structure is detected at or around the Agoudal shatter cones area indicates that if there was a local impact structure, it has been totally eroded or tectonically displaced, with the only remnant being the shatter cone-bearing beds, and possibly, the vertical to overturned layers that might correspond to disturbed layers below the crater or represent a remnant of the floor of an original impact structure.

## Thoughts about a Possibly Eroded Impact Structure—
with Reference to the Distribution of Shatter Cones

In general, impact structures are recognized from two main lines of evidence (1) the presence of evidence of shock metamorphic effects, and/or (2) the occurrence of remnants of the impacting projectile or geochemical traces thereof (cf. French and Koeberl 2010; Koeberl et al. 2012; Koeberl 2014). Shatter cones are the only known macro- to mesoscopic expression of shock metamorphism. Several impact structures, for which no discernible circular feature (e.g., Beaverhead, Santa Fe—Hargraves et al. 1990; Fackelman et al. 2008, respectively) have been identified on the basis of shatter cone occurrences, and some other impact structures (e.g., Vista Alegre, Brazil—Crósta et al. 2009, 2010) were also first identified from the presence of shatter cones. All known occurrences of shatter cones can be
<figure><img src="image_18.png" /><figcaption>Fig. 15. Vertical limestone bed with strong fracturing at the centimeter-scale, from the northeastern part of the main shatter cone occurrence at the central hill. Detail of Fig. 13.</figcaption></figure>

correlated with an estimated diameter of the impact crater inferred from geological or geophysical observations (Baratoux and Reimold 2016). This site at Agoudal is an exception to this rule, as no other observation related to the impact event, besides the presence of shatter cones, may be used to give a hint at the diameter of the former structure. The formation of shatter cones is only possible if rocks have been subjected to shock waves, that is, experienced shock pressures above a few GPa, considering the Hugoniot Elastic Limit in crustal rocks (Baratoux and Melosh 2003).

Shatter cones have been reported mostly from known impact structures larger than a few kilometers, but rarely from comparatively smaller impact craters. In detail, two subkilometer-sized impact structures have been cited as being associated with these typical, curved fractures decorated by divergent striations. One of these is the crater field of Sikhote Alin formed in 1947

<figure><img src="image_19.png" /><figcaption>Fig. 16. Rose diagram to represent the distribution of azimuth of the faults (or joints) at the Agoudal study site. The N70 to N110 orientations measured in the field are related to the regional High Atlas joint systems linked to the evolution of the main deformation due to the Atlas shortening (Frizon de Lamotte et al. 2008; Michard et al. 2011). They are not reported in the structural map of Fig. 10b. One major direction of jointing at N170 to N180 is clearly visible accompanied by many other randomly oriented joints.</figcaption></figure>

following the fall of many fragments (meteorite shower) of an iron meteorite. Twenty-three tons of material were collected by eight expeditions between 1947 and 1950 and in 1967–1970, and 120 craters and pits were identified, the largest of which had a diameter of 26.5 m. Shatter cones were claimed to be present in crater No. 4 of 19.2 m diameter (Krinov 1971). This would be the smallest impact crater associated with shatter cones. However, examination of the published images reveals no confirming evidence: the characteristic striations, for example, seem to be lacking. The author mentioned that shatter cones were poorly developed. It is also of note that criteria to recognize shatter cones were not discussed in those days with the same precision as today, and it is reasonable that geologists of the day had only limited knowledge of such features.

The second claim of association of shatter cones with small craters was made for the 45 m diameter, ~6000 yr old, Kamil crater (Fazio et al. 2014). The claim was supported by the presence of a melt layer on a putative shatter cone surface. However, the authors reported two objects, found in ejecta, and striations appear only on a small fraction of the sample surface of
<figure><img src="image_20.png" /><figcaption>Fig. 17. Some specimens of the Agoudal II AB iron meteorite. a) A complete irregularly shaped shrapnel of centimeter size of the Agoudal meteorite. b) Internal, still cohesive part of a highly weathered piece of the Agoudal meteorite. This sample was buried at 50 cm depth; there is still a traces of soil covering the meteorite.</figcaption></figure>

one of the two objects. The published close-up view of the set of striations is not entirely convincing: the striations are divergent but seemingly lack the typical rounded morphology associated with shatter cone striations. As just one single striated surface is shown, it is not possible to assess if striated fractures occur pervasively in the rock. No shatter cones were reported within the structure itself.

Thus, it appears that only impact structures larger than 2.5 km diameter contain undisputed shatter cones, as indicated by publications of photos with well-defined curved surface fractures with typical striations (from the database on shatter cones in Baratoux and Reimold (2016); Mishina Gora, 2.5 km: Masaitis et al. (1980); Goyder, 3 km: Haines: 1976; Flynn Creek, 3.8 km: Evenick et al. (2004); Steinheim, 3.8 km: Branco and Fraas (1905); Schmieder et al. 2010a, 2010b). Occurrences of shatter cones are generally buried in deeper layers of the target that are only exposed due to central uplift and/or erosion. Well-developed shatter cone surfaces have also been reported from small explosion experiments (Roddy and Davis 1977), from laboratory cratering experiments (Kenkmann et al. 2012; Wilk and Kenkmann 2016), and in meteorites (McHone et al. 2012; Ferrière et al. 2013).

Although the mechanism involved in shatter cone formation is still debated (e.g., Sagy et al. 2002, 2004; Baratoux and Melosh 2003; Wieland et al. 2006; Baratoux and Reimold 2016; Kenkmann et al. 2016), there is consensus that these fractures are formed as extensional cracks during the propagation of shock waves, and require pressure levels greater than 1 GPa. Such pressure intensity is clearly attainable in small impact structures, as long as the meteorite fragments collide with the ground at a velocity well above that of the speed of sound in the rock. From a theoretical standpoint, the formation of shatter cones in small impact structures is not impossible, as illustrated by

calculations made for the Agoudal case by Lorenz et al. (2015).

Therefore, no size limit for a possible eroded impact structure at Agoudal can be deduced from theoretical considerations applied to the presence of shatter cones. However, the extent and distribution of shatter cones must be commensurable with the dimension of the crater. Given the required intensity of shock to form shatter cones, they do not occur beyond the rim of the structure (except in the case of ejecta, but this does not apply here, as the shatter cone outcrops at Agoudal do not seem to be isolated blocks of ejecta). More precisely, shatter cone mapping studies on well exposed impact structures of various sizes from a few kilometers to a few tens of kilometers (e.g., Salameh et al. [2008] for Jebel Waqf as Suwwan; Crósta et al. [2012] and Yokoyama et al. [2015] for the Vargeão Dome; Hietela and Moilanen [2007] and Hasch et al. [2016] for Keurusselkä; Salisbury et al. [2008] for Lawn Hill; Kenkmann et al. [2011] for Serra da Cangalha; Holm et al. [2011] for Siljan; Stesky and Halls [1979] for Slate Islands; Hargraves et al. [1990] for Beaverhead) suggest that they generally occur within an area corresponding to 1/6 to 1/2 of the estimated original diameter of the impact crater. A circle of ~500 m diameter may be drawn to delineate the area of the Agoudal shatter cone occurrence (Figs. 5 and 10). Extrapolating observations at other impact structures, and considering that the currently known area of shatter cone occurrence at Agoudal corresponds to buried layers belonging to the central part of a single buried impact structure, the distribution of shatter cone locations suggests that the original diameter of the Agoudal crater would have been somewhere between 1000 (deduced and estimated from the approximate distance between most distant shatter cone outcrops of 700 m) and 3000 m. Although the lower limit is a robust constraint, the upper limit may be revised if new shatter cone exposures are
discovered, extending further the area of shatter cone occurrence. The range is lower but close to the smallest craters associated with unequivocal shatter cones, although it is of note that less than 20% of known impact structures of less than 3 km diameter are actually associated with (unambiguous) observations of shatter cones (Baratoux and Reimold 2016).

## Constraints on the Age and Size of a Presumed Agoudal Impact Structure from High Atlas Denudation Rates

The topography of the High Atlas is considered isostatically uncompensated and to result from several episodes of uplift associated with a combination of crustal shortening linked to the Africa-Eurasia convergence, with thermal-doming induced by lithospheric thinning or delamination, and as a result, as much as 1000 m of topography supported by underlying mantle (inter alia, Seber et al. 1998; Frizon de Lamotte et al. 2004; Zeyen et al. 2005; Teixell et al. 2003, 2005; Missenard et al. 2006, 2008; Miller and Becker 2014; Ramdani 1998; Boulton et al. 2014). The chronology of the uplift (and, therefore, denudation rates) remains a matter of debate (e.g., Beauchamp et al. 1999; Frizon de Lamotte et al. 2000; Tesón and Teixell 2008). Exhumation rates of 80–100 m Ma⁻¹ have been inferred from apatite fission-track thermochronology for the Neogene, whereas average denudation rates of 80 m Ma⁻¹ (0.08 mm yr⁻¹) have been estimated for the Anti-Atlas region (Ruiz et al. 2011). These values are consistent with slip rates on individual faults of 0.1 mm yr⁻¹ (100 m Ma⁻¹) for the quaternary period (Sébrier et al. 2006). The distribution and extent of shatter cones being compatible with a 1–3 km sized crater, we can infer an initial topography of ~1/10 of the crater diameter (100–300 m) (Melosh 1989). Considering a 0.8 mm yr⁻¹ denudation rate, it would have been possible to erase a crater of 1000–3000 m diameter in 1.25–3.75 million years (100–300 m of initial topography between rim and cavity center)—an age much older than the fall of the Agoudal meteorite estimated at <150 ka. It still remains possible—but unlikely—that locally higher erosion rates would allow erosion of a kilometer-sized structure in less than 150,000 yr, but then of course the meteorites would have been eroded away as well, as they were emplaced on the surface or just below (possibly down to a few meters deep). The shaded relief shown in Fig. 1 illustrates the fact that the shatter cones are localized in a wide valley that forms a relatively flat area that is surrounded by steep slopes. Although an incision rate of 10 cm yr⁻¹ would be considered exceptional, dedicated measurements of local incision rates would be needed to arrive at a definitive conclusion.

## Coincidental Association of Shatter Cones with Meteorites

Insights into the likelihood of association of an impact-deformed terrane and genetically unrelated meteorites can be drawn from the known terrestrial record of impact structures and meteorite falls. A world map of impact structures has been superimposed onto the known coordinates of more than 30,000 meteorite falls and finds (Fig. 18). The map illustrates that areas with a high density of impact structures are also characterized by a high density of recovered meteorites: the United States, Europe, North Africa, South Africa, and Australia. This situation is explained in part by the distribution of arid climatic zones, with both limited erosion of impact structures and slow weathering of extraterrestrial material, which is ideal for the preservation of meteorites and relatively small, recent impact structures. These data may be used to calculate statistics of distance between meteorite falls or finds and the nearest known impact structure. Figure 19 represents the number of meteorites as a function of distance from the nearest impact site normalized by the radius of that impact structure. Recovery of meteorites from within an impact structure is associated with distance normalized to impact structure radius less than 1 (12 cases). Most of these are actually true genetic associations of the impact structure with the recovered meteoritic material. However, at least two coincidental falls of meteorites within an impact structure have actually been reported. Some 76 g of LL6 ordinary chondrite were collected within the Aouelloul structure (Mauritania) (Fudali and Cressy 1976). The age of the fall was estimated at 0.07 Ma, whereby the age of the 390 m impact crater was given as 3.1 ± 0.3 Ma (Fudali and Cressy 1976). Another coincidence was reported from the crater field at Rio Cuarto (Argentina). The largest of these structures is strongly elongated (1000 × 4000 m) and is younger than 0.1 Ma. Some chondritic material from this structure seems to be related to the bolide responsible for the formation of these impacts (Koeberl and Schultz 1992), but its mass was not quantified. In addition to the collected H-chondrite, 62.7 g of eucritic material was found near the crater field, but the eucrite fall is not related to the formation of the impact field (Levine et al. 2007). In these two cases, the meteorites were found during field investigations of a known impact structure, whereas evidence for an eroded impact structure (shatter cones) was found during meteorite recovery. The figure demonstrates that a large number of meteorites is found within 5 radii of a known impact structure (but they are not genetically related). One may also consider that ~5000 meteorites (this number is
<figure><img src="image_21.png" /><figcaption>Fig. 18. Map of locations of impact structures (circles) and meteorite falls (red dots). The color of circles represents the ages of impact structures and the symbol size is proportional to the diameter of the impact structure. The source of the data for the meteorite falls and finds is the Database of the Meteoritical Society: http://www.lpi.usra.edu/meteor/. The projection of the map is a cylindrical one.</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>Fig. 19. Number of meteorite falls in the vicinity of impact structures (http://www.lpi.usra.edu/meteor/) represented as a function of the distance between the site of the fall and the nearest known impact site.</figcaption></figure>

based on the statistics from the Meteoritical Bulletin database while considering that at least half of the NWA meteorites comes from Morocco and that an even larger number of meteorites from Morocco have not been submitted to the nomenclature committee of the Meteoritical Society) have been found in about half of the area of Morocco (including the region of Agoudal), which leads to an average density of 0.02

events per km². This analysis suggests that the coincidence of a fall of meteorites with an impact structure is rare but certainly not impossible.

It is also important to consider impact craters in which fragments of the original impactor are still preserved. In most impact events, and especially in large ones, the projectile is entirely melted and/or vaporized (Pierazzo and Melosh 2000) and it is generally not possible to find bolide fragments associated with impact structures. There are various parameters that control the possibility to recover meteorite fragments, including impact velocity, impact angle, fragmentation in the atmosphere, and size of individual fragments, atmospheric entry of already fragmented material (e.g., Gritsevich and Koschny 2011; Bouquet et al. 2014), and subsequent weathering. In addition, meteorite fragments may be preserved within impact melt, as in the exceptional case of Morokweng (Maier et al. 2006; Jourdan et al. 2010). This case will not be considered in the following, and we will focus only on cases where meteoritic fragments were found on surface. As a consequence of the complexity of parameters controlling the occurrence of meteorite fragments associated with an impact structure, there is no correlation between the mass of recovered meteorites and the diameter of the structure (Fig. 20a). There are actually a large number
<figure><img src="image_23.png" /><figcaption>Fig. 20. a) Mass of meteorites (http://www.lpi.usra.edu/meteor/) versus impact structure diameter for impact structures associated with meteorite finds and where meteorites and impacts are genetically related. For crater fields (e.g., Sikhote Alin, Campo del Cielo), the error bar extends from the diameter of the largest structure to the diameter of circles having the same area as the entire crater field. b) For the same impact structures as in (a), the mass of meteorites was divided by an approximation of the mass excavated (a simple hemispheric shape of the crater has been used for this calculation) and plotted against the age of the fall (equal to the age of the impact). These data define a weak correlation represented by a field in light blue. The eroded Agoudal impact structure plots into that field for a broad range of possible diameters (1–3 km).</figcaption></figure>

of falls/finds approaching—or significantly greater than
—one ton that are not associated with known impact
structures, either because the structure was rapidly
eroded, or because it never formed as a consequence of
insufficient terminal impact velocity after atmospheric
fragmentation and drag. Therefore, there is no
correlation between the recovered mass of meteorites
and the actual mass of the bolide responsible for the
spatially associated impact structure and that was
vaporized or molten.

However, when the age of the impact structure is
considered, it is possible to detect a weak correlation
between the mass of meteorites normalized by the
volume of the crater with the age of the crater.
Figure 20b represents the mass of meteorites divided by
the excavated mass of the crater, assuming a
hemispherical shape and a density of 3000 kg m⁻³, as a
function of the age (the assumed shape and density are
not important in this calculation that is just intended to
illustrate the correlation). This weak correlation reflects
the potential of preservation, which depends on the
intensity of duration of weathering. Considering that
shatter cones at Agoudal occur in an area ~1 km in
diameter and a recovered mass of 2 tons of meteorites,

it is of note that the case of Agoudal would plot within
the field of impact craters genetically related to
meteorites. However, as noted before, there are also
numerous falls greater than 1 ton that are not
associated with impact craters. In addition, it should be
noted that meteorite fragments at Agoudal were all
found at or near the surface and only a few shrapnel
shaped meteorites were collected—which would be
expected if this meteorite was related to an impact crater.
Therefore, Fig. 20b cannot be used to support or
exclude a genetic relationship between the shatter cones
and the Agoudal meteorite. In the case of Agoudal, the
size of the original impact site and the “young” age of
the meteorite related to the time needed for the erosion
of the material and exposure of the shatter cone-bearing
crater floor of even such a small crater are not in favor
of a genetic relationship. Previously reported
coincidental associations of impact structures and
meteorite falls indicate that the Agoudal conundrum
would be actually the third coincidental meteorite fall
within a known impact structure, but it is the first time
that evidence likely pointing at an eroded impact
structure (shatter cones) was reported subsequently to
the finding of the meteorites.
## CONCLUSIONS

Agoudal is the first site with impact evidence found in Morocco; it is confirmed by the presence of several outcrops with excellent exposures of impact-characteristic shatter cones. These outcrops are distributed within an area of ~500 × 300 m. The area of shatter cone occurrence overlaps with the strewn field of the Agoudal iron meteorite, which led to a debate regarding a possible genetic link between the meteorite and the shatter cone occurrence. The shatter cone area is surrounded in the northern part by fractures affecting a vertically oriented layer of marly limestone accompanied by small blocks (between fractures) with meter-scale displacements against each other. In the main shatter cone hill, structural observations are relatively chaotic and the shatter cone occurrences are highly fractured. Fracture orientation measurements show the High Atlas signature accompanied by one important fracture direction at N170 to N180 and other, subsidiary trends, which do not show any preferential distribution.

Considering the distribution of shatter cones together with the knowledge from the terrestrial impact record and assuming that the Agoudal shatter cone occurrence belongs to one single impact structure leads us to conclude that the pre-erosion diameter of this impact structure must have been larger than at least 1 km, but could have been as large as 3 km. A structure of this size would need at least 1 million years to be eroded, when considering a denudation rate of less than 0.1 cm yr⁻¹ in the Atlas region. This age is one order of magnitude larger than the age of the Agoudal iron meteorite (105 ± 40 kyr). If the impact structure was even larger, an even longer time period would have been necessary.

As a consequence, we conclude that the Agoudal iron meteorite and the shatter cones are not coeval. Such coincidental events do not appear unlikely, from a statistical point of view. The case of Agoudal may exemplify future discoveries of impact sites that are severely eroded, similar to other large, deeply eroded impact structures (Santa Fe, Beaverhead) for which no circular structure remains. Great challenges are posed by the scarcity of information that is available for such structures, whereas identification and determination of the characteristics of such impact events would remain essential to pursue the goal of documenting the collisional history of the Earth with extra-terrestrial objects, and to better constrain the risks associated with impacts. A better understanding of the shatter cone formation mechanism would allow a better use of these objects to derive quantitative information about the bolide and impact structure parameters.

*Acknowledgments*—Field work was partially supported by the CMIFMP Volubilis (MA/11/252) program. Special thanks to Professor Fouad El Kamel from the Hassan II University of Casablanca, Faculty of Sciences Ain Chock, for his help provided on structural data treatment, as well as students Said Benchelha, Taha Shisseh, and Rabii Chaarani, who helped with ArcGis applications. WUR's research is supported by the Museum für Naturkunde Berlin. DB's position at the IRD-Dakar/Institut Fondamental d'Afrique Noire is supported by the Institut de Recherche pour le Développement. Associate editor M. Poelchau and reviewers L. Pittarello and A. Crósta are thanked for their detailed reviews and constructive comments.

*Editorial Handling*—Dr. Michael Poelchau

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