# Bosumtwi impact structure, Ghana: Evidence for fluidized emplacement of the ejecta

David Baratoux, Cheikh Ahmadou Bamba Niang, Wolf Uwe Reimold, Marian

Selorm Sapah, Mark Walter Jessell, Daniel Boamah, Gayane Faye, Sylvain

Bouley, Olivier Vanderhaeghe

► To cite this version:

David Baratoux, Cheikh Ahmadou Bamba Niang, Wolf Uwe Reimold, Marian Selorm Sapah, Mark Walter Jessell, et al.. Bosumtwi impact structure, Ghana: Evidence for fluidized emplacement of the ejecta. Meteoritics and Planetary Science, 2019, 54 (10), pp.2541-2556 (IF 2,719). 10.1111/maps.13253. hal-02335524

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# Bosumtwi impact structure, Ghana: Evidence for fluidized emplacement of the ejecta

David BARATOUX<sup>1,2*</sup>, Cheikh Ahmadou Bamba NIANG<sup>2,3</sup>, Wolf Uwe REIMOLD<sup>4,5</sup>, Marian Selorm SAPAH<sup>6</sup>, Mark Walter JESSELL<sup>1,7</sup>, Daniel BOAMAH<sup>8</sup>, Gayane FAYE<sup>9</sup>, Sylvain BOULEY<sup>10</sup>, and Olivier VANDERHAEGHE<sup>1</sup>

*(Received 30 May 2018; revision accepted 27 December 2018)

**Abstract**—The about 10.5 km diameter Bosumtwi impact crater is one of the youngest large impact structures on Earth. The crater rim is readily noticed on topographic maps or in satellite imagery. It defines a circular basin filled by water (Lake Bosumtwi) and lacustrine sediments. The morphology of this impact structure is also characterized by a circular plateau extending beyond the rim and up to 9–10 km from the center of the crater (about 2 crater radii). This feature comprises a shallow ring depression, also described as an annular moat, and a subdued circular ridge at its outer edge. The origin of this outermost feature could so far not be elucidated based on remote sensing data only. Our approach combines detailed topographic analysis, including roughness mapping, with airborne radiometric surveys (mapping near-surface K, Th, U concentrations) and field observations. This provides evidence that the moat and outer ring are features inherited from the impact event and represent the partially eroded ejecta layer of the Bosumtwi impact structure. The characteristics of the outer ridge indicate that ejecta emplacement was not purely ballistic but requires ejecta fluidization and surface flow. The setting of Bosumtwi ejecta can therefore be considered as a terrestrial analog for rampart craters, which are common on Mars and Venus, and also found on icy bodies of the outer solar system (e.g., Ganymede, Europa, Dione, Tethys, and Charon). Future studies at Bosumtwi may therefore help to elucidate the mechanism of formation of rampart craters.

## INTRODUCTION

About 195 impact structures have been identified on the surface of the Earth—far less than the number of impact structures on other planetary surfaces. This is

explained by the resurfacing rates of tectonically or volcanically active regions, by the geologically fast subduction of oceanic crust covering one-third of the Earth's surface, and by high erosion rates. Erosion rates at the Earth's surface range from 0.1 m Ma⁻<fnref n="1" /> to

<footnote n="1">¹Géosciences Environnement Toulouse, UMR5563, CNRS, Université de Toulouse & IRD, 14, Avenue Edouard Belin, 31400 Toulouse, France</footnote>

<footnote n="2">²Institut Fondamental d'Afrique Noire Cheikh Anta Diop, Dakar, Senegal</footnote>

<footnote n="3">³Département de Géologie de l'Université Cheikh Anta Diop, Dakar, Senegal</footnote>

<footnote n="4">⁴Museum für Naturkunde—Leibniz Institute for Evolution and Biodiversity Science, Invalidenstrasse 43, D-10115 Berlin, Germany</footnote>

<footnote n="5">⁵Institute of Geosciences, Laboratory of Geodynamics, Geochronology and Envoironmental Science, University of Brasília, Brasília, Brazil</footnote>

<footnote n="6">⁶Department of Earth Science, University of Ghana, Accra, Ghana</footnote>

<footnote n="7">⁷Centre for Exploration Targeting, School of Earth Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia</footnote>

<footnote n="8">⁸Geological Survey Department, Accra, Ghana</footnote>

<footnote n="9">⁹Institut des Sciences de la Terre, Laboratoire de Télédétection Appliquée, Université Cheikh Anta Diop, Dakar, Senegal</footnote>

<footnote n="10">¹⁰GEOPS — Géosciences Paris Sud, Univ. Paris-Sud, CNRS, Université Paris-Saclay, Rue du Belvédère, Bât. 504-509, 91405 Orsay, France</footnote>

<footnote>*Corresponding author. E-mail: david.baratoux@ird.fr</footnote>
<figure><img src="image_2.png" /><figcaption>Fig. 1. Ages of confirmed terrestrial impact structures as a function of their diameter (source: Impact crater database, http://www.passc.net/EarthImpactDatabase, accessed last on 22 October 2018). Six relatively large and recent impact structures, highlighted in red (Bosumtwi) and orange, have the potential to preserve ejecta deposits. For impact structures plotted in gray, only a minimum or a maximum age is available. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

100,000 m $Ma^{-1}$, with typical values for the activity of drainage basins on the order of 10 to 1000 m $Ma^{-1}$ (Portenga and Bierman 2011; Hecht and Oguchi 2017). For comparison, the present erosion rate on Mars has been estimated at $10^{-40} \mu m yr^{-1}$ (Golombek and Bridges 2000). These erosion rates are considered valid for most of the Martian history, i.e., from 3.7 Ga until present. Higher erosion rates, on the order of $1–100 m Ma^{-1}$ have only been suggested for the Noachian period (before 3.7 Ga). As a consequence, almost 400,000 impact craters larger than 1 km in diameter can be identified on the Martian surface (Robbins and Hynek 2012).

Most of the recognized impact structures on Earth are buried under sediments or have been deeply eroded, and do no longer preserve their initial morphology. Ejecta deposits, which are well identified on other planetary surfaces, are mainly preserved for recent terrestrial impact structures. This is the case, for instance, for the Kamil impact structure in Egypt, 45 m in diameter, which is younger than 5000 yr (Folco et al. 2011). Meteor crater, with a diameter of 1200 m and an age of 50,000 yr, is another example of a recent impact structure with preserved ejecta. For the latter example, the extent of the ejecta layer is apparent in topographic data (Wulf and Kenkmann 2018), whereas the composition and source of the ejecta may be mapped based on multispectral satellite imagery and knowledge of the local stratigraphy (Ramsey 2002).

Most of the large impact structures are older, and their ejecta were completely eroded after a few hundred thousand years to a few million years. The occurrence of preserved ejecta deposits depends on several factors,

including size and age of the structure, and climatic and tectonic histories. When plotting the ages of confirmed impact structures as a function of their diameter, six large (>5 km) and recent (<15 Ma) impact structures, namely Ries, Zhamanshin, Karla, El'gygytgyn, Bigach, and Bosumtwi, are identified as special cases. Despite the fact that climatic and tectonic factors should be considered together in order to discuss erosion history, the fact that these structures appear as outliers in Fig. 1 suggests that they have a unique potential for exposure of preserved ejecta deposits. Little information is available for Bigach, but Masaitis (1999) reported several outcrops of ejected breccias outside the crater. Field work and field mapping have been delimited to the central part of the Karla structure (Masaitis et al. 1980), which appears to be largely eroded and covered by quaternary sands. There is no information available on ejecta outcrops or ejecta morphology. Garvin and Schnetzler (1992) noted that the topography of Zhamanshin would suggest much higher erosion rates in comparison with Bosumtwi, which has a similar age and a comparable diameter. Detailed analysis of the structure and morphology of El'gygytgyn suggested that the ejecta of this crater have been entirely eroded (Gurov et al. 2007). The Ries impact crater is about 26 km diameter. The age of this impact structure is about 14.8 Ma ($14.83 \pm 0.15$ Ma according to Jourdan et al. 2012; and $14.808 \pm 0.021$ Ma according to Schmieder et al. 2018; based on an $^{40}$Ar/$^{390}$Ar age for moldavite tektites). Sturm et al. (2013) attempted a 3-D reconstruction of the thickness of the Bunte Breccia deposits and demonstrated that the southernmost part of the ejecta deposits of the Ries crater was partially preserved.
The morphology of the Bosumtwi impact crater in Ghana is characterized by a well-defined near-circular basin (filled by the water and lacustrine sediments of Lake Bosumtwi) surrounded by a prominent crater rim that rises to 200–250 m above the surface of the lake. An outer circular plateau was mapped by Reimold et al. (1998) and discussed by Koeberl and Reimold (2005). The plateau extends to about 10 km from the center of the structure, where it is delimited by a subtle circular ridge. This feature was previously noted and discussed by Jones et al. (1981) and Wagner et al. (2002), exclusively from remote sensing observations. The nature of this feature has remained debated, and the authors of these articles mentioned that this outermost feature could either correspond to the limit of an ejecta deposit, or to a ring fault. Wulf and Kenkmann (2017) noted the similarity between the morphology of the Bosumtwi impact crater and Martian rampart craters, and favored the hypothesis that the circular plateau is a partially eroded ejecta layer. Reaching a definitive conclusion is difficult because only a few large impact structures with well-preserved ejecta blankets are available on Earth, and available for comparison. Moreover, the Bosumtwi crater has been subjected to a tropical climate with intense precipitation, but the region is tectonically stable, and the intensity of weathering and/or of erosion at Bosumtwi is not well documented. At the regional scale, denudation rates of 5–15 m Ma⁻¹ have been estimated based on ages and interpolations of lateritic paleosurfaces (Beauvais and Chardon 2013), but it is not clear whether these values apply to the Bosumtwi impact structure, considering the peculiarity and age of its topography with respect to other landforms of the Ashanti belt. It is also difficult to determine, based on topographic and imagery data only, if the plateau is a consequence of the degradation of the crater, which may have had a different morphology than present, or if it is the remnant of an original feature resulting from the formation of the impact crater.

Our investigation tests the hypothesis that the circular plateau and outermost ring are ejecta facies. It comprises an analysis of denoised SRTM data (Yamazaki et al. 2017), field observations, and airborne radiometric surveys that provided a map of K, Th, and U concentrations in the near-surface material.

This article is divided into five sections. The next section presents background information about the geology of the Bosumtwi impact structure and the K, Th, U concentrations of the main lithologies exposed in the Bosumtwi area as a reference to interpret the radiometric map. The next section (Morphologicla Characteristics of Ballistic Versus Rampart Ejecta) summarizes the morphology of ejecta focusing on the features that distinguish ballistic ejecta from rampart ejecta (or rampart craters). The results of the

morphological analysis of the Bosumtwi impact structure are presented in the Morphological Analysis of the Bosumtwi Impact Crater section, and the interpretation of radiometric data and field observations in the Insights from Airborne Radiometrics and Field Observations section. The last section is a discussion on the value of Bosumtwi impact ejecta to better understand the respective roles played by atmospheric and subsurface volatiles during ejecta emplacement (e.g., Barnouin-Jha and Schultz 1998; Baratoux et al. 2002; Baloga et al. 2005; Barnouin-Jha et al. 2005).

## GEOLOGICAL BACKGROUND AND K, TH, U CONCENTRATIONS

Geological mapping around Lake Bosumtwi has been carried out by Junner (1937), Woodfield (1966), Moon and Mason (1967), Jones et al. (1981), and Koeberl and Reimold (2005). A simplified geological map of the impact structure and its surroundings is given in Fig. 2. Geological mapping in this region is challenged by several obstacles, including dense tropical rain forest, and deep lateritic profiles, which are commonly developed in the southern part of Ghana. Outcrops are more frequent at the crater itself than throughout its surroundings due to its rough topography, which contrasts with the smooth lateritic plateau surrounding the structure. Good outcrops are also found along stream-beds, at villages, and along recent road cuts. A few meters below the lateritic cover, that was likely present around 1 Ma ago, the pre-impact lithologies are metasediments in lower greenschist facies (greywacke, quartzitic greywacke, meta-tuffs, phyllites, shales, and schists) of the 2.1–2.2 Ga Birimian Supergroup (cf. Wright et al. 1985; Leube et al. 1990). To the southeast of the structure, the hills forming the Obuom Range are composed of basic intrusive and Birimian metasediments. Small exposures of Proterozoic granitic intrusions have also been mapped within the structure. Two families of dikes of dolerite and amphibolite have been noted around the crater. The dikes oriented N–S have an age of 867 ± 16 Ma and the family oriented NE–SW has an age of 915 ± 8 Ma (Baratoux et al. 2018). In the immediate environs of the crater, lithology is dominated by greywacke, sandstone/quartzitic rocks, and minor mica schist. Most recent lithologies include the lacustrine sediments, the breccias, and suevite directly associated with the impact and excavation of the crater. Structural observations indicate that the Birimian sediments are generally folded dipping at high angles with a general NE–SW strike. Strikes and dips are modified around the crater rim, as the result of impact-related deformation (Reimold et al. 1998).
<figure><img src="image_3.png" /><figcaption>Fig. 2. Simplified geological map of the Bosumtwi impact structure (modified after Koeberl and Reimold 2005). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

<table><caption>Table 1. K, Th, U concentrations of the main lithologies exposed at the Bosumtwi impact structure (for each lithology, the left column provides the average value and 1 sigma standard deviation, and the right column indicates the full range of observed concentrations. The data are taken from Koeberl et al. (1998).</caption><thead><tr><th></th><th colspan="2">Shale</th><th colspan="2">Greywacke—Phyllite</th><th colspan="2">Granite dike</th><th colspan="2">Pepiakese granite</th></tr></thead><tbody><tr><td>K (wt%)</td><td>2.39 ± 0.13</td><td>2.19–2.55</td><td>1.49 ± 0.50</td><td>0.71–2.26</td><td>1.60 ± 0.13</td><td>0.56 ± 0.36</td><td>0.22–1.05</td></tr><tr><td>Th (ppm)</td><td>4.36 ± 0.09</td><td>4.23–4.48</td><td>3.94 ± 0.56</td><td>2.99–4.47</td><td>3.10 ± 0.27</td><td>2.21 ± 2.09</td><td>0.71–5.17</td></tr><tr><td>U (ppm)</td><td>1.69 ± 0.16</td><td>1.43–1.85</td><td>1.35 ± 0.15</td><td>1.18–1.62</td><td>1.75 ± 0.11</td><td>0.74 ± 0.53</td><td>0.23–1.47</td></tr></tbody></table>

As a reference for the interpretation of airborne radiometric data, we have summarized in Table 1 the concentrations of K, Th, and U in the lithologies occurring at and around the impact structure. These data have been extracted from geochemical analyses published by Koeberl et al. (1998). The K, Th, U concentrations of the different lithologies show a relatively limited range of variations. The shale is slightly enriched in K and Th with respect to other lithologies, and the Pepiakese granite (which has a limited occurrence in the Northeastern part of the rim) is comparatively poor in K, Th, and U. The full range of concentrations observed for each lithology suggests that it would not be feasible to distinguish the different types of rocks exposed at the crater based on K, Th, and U concentrations alone.

## MORPHOLOGICAL CHARACTERISTICS OF BALLISTIC VERSUS RAMPART EJECTA

Before the era of exploration of Mars by orbiters, the morphology of impact ejecta was known from the observation of lunar impact craters. The topography (or thickness) and granulometry of these ejecta decrease progressively away from the crater rim. The texture and topography of these ejecta are explained by the ballistic trajectory of rocky fragments produced during the excavation flow of the crater. McCauley (1973) was the first to recognize the existence of a different class of impact ejecta on Mars, based on Mariner 9 images. The distinctive features of these ejecta include a fluidized appearance, the occurrence of grooves and scarps, and
<figure><img src="image_4.png" /><figcaption>Fig. 3. Typical morphology of a Martian rampart crater with its distal ridge. The example is an impact crater centered at 16.4°N 294.6°E. Source: THEMIS-IR (Mars Odyssey) daytime mosaic. Lower right inset: elevations from Mars Orbiter Laser Altimeter Data corresponding to the profile indicated by the white line.</figcaption></figure>

one or several sinuous or lobate escarpments (rampart) (Fig. 3). Carr et al. (1977) formulated the now most widely accepted hypothesis that the rampart ejecta morphology is a consequence of surface flow of ejecta fluidized by subsurface volatiles. Indeed, a fluidization mechanism is necessary to explain the formation of a distal rampart which cannot be obtained by purely ballistic trajectories (Carr et al. 1977; Mouginis-Mark 1978; Baratoux et al. 2005; Boyce and Mouginis-Mark 2006; Mouginis-Mark and Baloga 2006). The formation of the distal rampart appears as a natural consequence of a gradual ground hugging flow with a resistance to flow that is proportional to the volumetric flow rate (Bingham-plastic behavior, Baloga et al. 2005). The possible fluidization mechanisms are still debated and involve either subsurface volatiles (Carr et al. 1977; Gault and Greeley 1978; Mouginis-Mark 1978, 1979, 1981, 1987) or atmospheric volatiles (Schultz and Gault 1979; Schultz 1992; Barnouin-Jha and Schultz 1996, 1998). Alternatively, the rampart may also result from the distal sedimentation of particles entrained in vortices, when the intensity of the vortices is not sufficient to support particle entrainment (Barnouin-Jha and Schultz 1998).

The geographic variations of occurrences and morphological parameters (ejecta extent, sinuosity) of rampart ejecta on Mars support the hypothesis of subsurface volatiles and may be used to decipher

changes in concentrations or depth of subsurface ice (Mouginis-Mark 1979; Schultz 1992; Barnouin-Jha and Schultz 1998; Barlow et al. 2000; Baratoux et al. 2002; Barlow and Perez 2003). However, the presence of rampart ejecta at the surface of Venus demonstrates that similar morphologies may be formed under a dense atmosphere in a water-free subsurface (Barnouin-Jha and Schultz 1996, 1998). Other ejecta morphologies similar to the rampart craters on Mars have been identified on icy satellites including Ganymede (Boyce et al. 2010), Europa, Dione, Tethys, and Charon (Robbins et al. 2018). These observations support the hypothesis that subsurface ices are responsible for fluidization during ejecta emplacement. The debate about the formation of various morphologies of ejecta in the solar system has not been settled. The formation of a distal rampart marking the edge of a continuous ejecta blanket is a distinctive feature, which cannot be explained by ballistic deposition alone. It necessarily requires ejecta fluidization and surface flow—irrespective of the origin and nature of the fluidizing agent (gas or liquid), and of the relative proportion of fluid and solid particles.

## MORPHOLOGICAL ANALYSIS OF THE BOSUMTWI IMPACT CRATER

### Topographic Data Processing

The morphological analysis of Bosumtwi is based on the denoised SRTM topographic data released by Yamazaki et al. (2017). The resolution of these data is 90 m/pixel, which is lower than that for SRTM1 data (1 arc second per pixel, or 30 m/pixel), but the denoised data have better vertical accuracy. Fifty-eight percent of land areas have been mapped with a vertical accuracy better than 2 m. Also, these data are more appropriate for detailed topography analysis in areas of low relief, or in areas that are covered by forest or dense vegetation (Yamazaki et al. 2017). The topography of Bosumtwi is analyzed based on a shaded relief map (Fig. 4), on several radial topographic profiles (Fig. 5), and on a roughness map calculated from denoised SRTM data (Fig. 6).

The shaded relief map was generated with a sun azimuth of 45° (NE illumination) and a sun elevation of 25° using the ENVI software. The shaded relief map was then overlain onto a color-coded topographic map (Fig. 4).

Radial topographic profiles were extracted for various azimuths (see Fig. 4 for positions of the radial profiles and Fig. 5 for the profiles), with the exception of the southeastern part of the crater structure, which is perturbed by the relief associated with the
<figure><img src="image_5.png" /><figcaption>Fig. 4. Shaded relief map of the Bosumtwi impact crater and environs overlain onto color-coded topography. Map projection UTM zone 30N, upper left coordinate: 660 872.393E (–1°32′41.06″), 734 674.385N (6°38′41.36″), lower right coordinate: 690 842.393E (–1°16′22.60″), 704705.385N (6°22′22.60″). Tracks of topographic profiles in Fig. 5 are indicated with dotted white lines. Black dots correspond to outcrop locations with corresponding illustrations in Figs. 9–11. Borehole locations BH1 and BH3 (Boamah and Koeberl 2002a, 2002b) are also given. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

metavolcanics of the Birimian supergroup (Koeberl and Reimold [2005] and references therein).

There is no universal definition of roughness (Kreslavsky et al. 2013), and it is therefore necessary to describe the roughness estimator used in this study. Roughness R is quantified here using the interquartile scale variation of topography in a sliding window (Aharonson et al. 1998):

$$R = \frac{N}{0.673 * (2N - 1)} (Q_3 - Q_1) \quad (1)$$

where $Q_i$ is the elevation of the ith quartile point and N is the number of points. The term 0.673 is a normalization factor corresponding to a normal distribution (i.e., R = 1 for a normal distribution). The roughness is calculated

for scales of 990 m, 2970 m, 8910 m, corresponding to window widths of 11, 31, and 91 pixels, respectively. The 990 m value corresponds to the smallest possible roughness scale given the resolution of the SRTM data (i.e., the roughness is calculated based on the statistics of elevations of 121 pixels). The largest roughness scale was chosen to be comparable to the size of the crater, and the roughness is calculated for an intermediate roughness scale, with a geometric progression of the scales. Odd numbers of pixels are chosen, so the roughness may be calculated for the center of each window. The three roughness maps are represented as a composite RGB image, with small-scale roughness (990 m) assigned to the red channel, intermediate-scale roughness (2970 m) assigned to the green channel, and large-scale roughness (8910 m) assigned to the blue channel (Fig. 6).
<figure><img src="image_6.png" /><figcaption>Fig. 5. Topographic profiles corresponding to the profile tracks marked in Fig. 4. P2–P5 profiles are offset by steps of 150 m for clarity.</figcaption></figure>

## Topographic Data Interpretation

The previously identified main components of the topography (Wagner et al. 2002), including the crater rim, the circular plateau extending up to 9–10 km from the center of the crater, and the second subdued circular feature beyond 15 km, are easily seen in both the shaded relief (Fig. 4) and roughness maps (Fig. 6). The crater rim is characterized by high roughness values at all scales. The circular plateau, with elevations of only a few tens of meters above the surrounding terrain, is characterized by an annulus of comparatively smooth topography.

The radial topographic profiles (Fig. 5) highlight the details of the topography. The plateau is composed of a ring moat, where topography is smooth. The elevation increases further away from the rim, and the outer edge of the ring moat is represented by a circular topographic ridge, which is asymmetric: the inward slope of the ridge is generally steeper than the outward slope. The presence of a ring moat and outer ridge at 9–10 km (~2 crater radii) from the crater center was previously noted by Wagner et al. (2002). The ridge is discontinuous, and is not observed in a small angular sector in the northwestern part of the crater structure, where it may have been eroded. It cannot be followed either in the southeastern part of the structure, where

topographic highs associated with Birimian metavolcanics and the Obuom Range are present. The outer part of the ridge is characterized by a rough topography at the small scale and appears in red on the roughness map (see white dotted lines in Fig. 6). The roughness of the outer part of the ridge is indicative of more intense erosion, associated with a possibly denser drainage pattern. The shaded relief (Fig. 4) and roughness maps (Fig. 6) indicate a dissected terrain on the outer part of the ridge, with a succession of small hills of decreasing amplitudes (Fig. 4). Such a rough topography is likely shaped by currently active erosion.

The terrain becomes much smoother beyond the ridge, which could be the consequence of deposited material having been eroded from the dissected terrain. Alternatively, this smooth area may be the consequence of erosion of a pre-existing layer of ejecta. However, there is seemingly no clear field evidence of ejecta material beyond the outer ridge, judging from previous works, with only two outcrops with unclassified breccia having been mapped by Koeberl and Reimold (2005) within this smooth area.

Farther away, the roughness at the intermediate scale increases and reaches typical signatures for lateritic plateaus encountered in West Africa (Metelka et al. 2018). The rough area corresponding to the Birimian metavolcanics in the southeastern part of the crater structure is not discussed here. Those specific signatures are more likely dominated by the erosion of a significant pre-impact relief (Obuom Range), rather than by impact-related processes.

The morphology of such an ejecta layer is very similar to that of rampart ejecta craters on Mars. Our analysis of topographic data would, therefore, concur with the previously formulated hypothesis (Wagner et al. 2002), which has also been recently advocated based on topography data and satellite imagery (Wulf and Kenkmann 2017), that this circular plateau, composed of a ring moat and asymmetric outer ridge, corresponds to a partially eroded rampart ejecta deposit.

However, alternative hypotheses have been proposed and cannot be ruled out based on remote sensing and morphological analyses only. The ring moat and ridge might be features inherited from the impact event, or could be the result of postimpact erosion. For instance, Wagner et al. (2002) suggested that preferential erosion of the ejecta layer could be responsible for the ring moat. Assuming the plateau is an ejecta layer, the presence of the circular ridge is actually a distinctive feature, which is not observed for ejecta which are exclusively emplaced by ballistic trajectories. In the case of ballistic deposition, the ejecta topography is a continuously decreasing function
<figure><img src="image_7.png" /><figcaption>Fig. 6. Roughness map for the Bosumtwi impact structure overlain onto a shaded relief image of the crater. The stretching parameters for the roughness values are, respectively, 0.5–4 m for the red channel, showing the roughness calculated with a cell of 990 m; 2–6 m for the green channel, showing the roughness calculated with a cell of 2970 m; and 3–10 m for the blue channel, showing the roughness calculated with a cell of 8910 m. The rough area corresponding to the outer ridge (or rampart) is emphasized by the dotted white outline. Map projection as in Fig. 4. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

of the distance from the center of the crater. The formation of a topographic ridge at the limit of the ejecta layer implies ejecta emplacement by non-ballistic processes, including surface flow (see the Morphological Characteristics of Ballistic Versus Rampart Ejecta section). The presence of a circular ridge along the edge of the Bosumtwi ejecta deposit would therefore indicate that subsurface and/or atmospheric volatiles were involved during ejecta emplacement.

However, outer ridges have also been described at other terrestrial impact structures in the size range from 10 to 24 km—in particular, at Elgygytgyn (Gurov and Yamnichenko 1995) and the Ries crater (Johnson et al. 1964). The nature of these outer ridges has been debated and possible explanations are (1) that it represents the actual crater rim, (2) a ring fault formed

during the crater modification stage (assumed also for the small BP crater—Koeberl et al. 2005), and (3) the outer limit of a continuous ejecta blanket modified by postimpact erosion. In order to demonstrate the rampart ejecta hypothesis and rule out other hypotheses in the case of the plateau and outer ridge observed at Bosumtwi, it is therefore necessary to carefully determine whether:

1. the plateau and ridge are essentially composed of ejecta material (such as ejecta blocks, impact melt, suevite), and

2. the circular ridge was present at the time of crater formation and is not the result of the postimpact erosional history.

The following section addresses these two points based on previous and new field observations and analysis of an airborne radiometric survey.
# INSIGHTS FROM AIRBORNE RADIOMETRICS AND FIELD OBSERVATIONS

## Data Processing

Two airborne radiometric surveys were provided by the Geological Survey of Ghana for this study. One of them (Aerodat) was acquired with a gridded line spacing of 200 m. It is fully calibrated and provides absolute values of surface concentrations of K (wt%), Th (ppm), and U (ppm). Unfortunately, only half of the Bosumtwi impact crater is covered by this survey. A second survey (Hisense) had a grid line spacing of 400 m and covers the entire impact structure, but it is not calibrated. The two airborne surveys are used to generate a gridded data set of K, Th, and U concentrations at a resolution of 50 m/pixel. For this purpose, the following steps were followed. Both surveys were gridded at 50 m/pixel. The Hisense survey was then warped on the Aerodat survey using a set of manually selected tie points between the two surveys. This step is necessary because a slight spatial misregistration was noted between the two surveys based on the examination of overlapping areas. An empirical linear calibration curve was determined from all corresponding pixels within a region of ~1000 km² surrounding the impact crater. The following calibration curve is then applied to the non-calibrated (DN) gridded values of the Hisense survey to produce a map of K, Th, and U concentrations:

$$K(\text{wt}\%) = 0.01369 * \text{DN} - 0.02286 \quad (2)$$

$$\text{Th (ppm)} = 0.2387 * \text{DN} - 0.3575 \quad (3)$$

$$\text{U (ppm)} = 0.1457 * \text{DN} - 0.1853 \quad (4)$$

A mosaic of the two surveys was then generated. The higher resolution is kept for the regions with overlapping data from the two surveys. The resulting map of K, Th, U concentrations is provided as a ternary grid (Fig. 7).

## Analysis of Airborne Radiometrics

The most prominent features observed in the radiometric data are two rings of elevated concentrations in K, as noted before by Pesonen et al. (2003) and Boamah and Koeberl (2002a, 2002b). Thorium and U are also good tracers of geological processes, but their concentrations are low in the metasediments. Concentrations of a few ppm for Th, and less than 2 ppm for U are suggested from the radiometric data, and these values have been confirmed

by ground measurements with a portable gamma-ray spectrometer (Niang et al. 2018). Detection limits for Th and U are on the order of a few tenths of ppm with these techniques. The Th and U data are, therefore, noisy and do not show any circular pattern that could be attributed to the impact. We will therefore focus in the following on the annular K pattern.

Variations in the K concentration are also relatively modest. The lowest values are about 1 wt%, whereas highest values are slightly above 2 wt%. The most enriched area in K is associated with the crater rim. The second K-rich area is clearly associated with the outer ridge. The moat is characterized by lower K concentrations. The correlation between elevated concentrations in K and roughness may be noted by comparing Figs. 6 and 7 (rough areas are delineated by a white dotted line). Figure 8 displays six radial topographic profiles and K concentration profiles. These profiles demonstrate the strong correlation between the concentration of K and the crater rim. The ring moat, characterized by a smooth topography, is associated with relatively lower concentration of K. The second peak of concentration of K coincides with the ridge, and more precisely with the outer part of the ridge, corresponding to the dissected terrain with a rough topography.

## Field Observations

Boamah and Koeberl (2002a, 2002b) provided geochemical analyses of 83 soil samples and confirmed that the high radiometric K signal is associated with elevated concentration of K in the soils. They suggested that the high concentration of K may reflect mobilization of K due to hydrothermal activity related to the impact event or may be explained by variation of the concentrations of K in the source rocks, including the metasediments themselves, or considering the presence of K-rich granitic lithologies (as also discussed by Wagner et al. 2002). Niang et al. (2018) carried out new detailed mapping of K, Th, U concentrations with a portable gamma-ray spectrometer. These observations confirm the range of concentrations obtained from the airborne survey and provide information about the lithology of K-rich and K-poor areas. An important observation was made that concentrations at the ~1 wt% level are generally associated with lateritic cover (example of exposed surface for K-poor areas also shown here in Fig. 9), whereas values at ~2 wt% correspond to outcrops of metasediments (examples of exposed surfaces for K-rich areas are also shown in Fig. 10). Saprolith has been observed within the ring moat (see Fig. 11) in association with low concentrations of K. Outcrops of metasediments are
<figure><img src="image_8.png" /><figcaption>Fig. 7. K, Th, and U concentration map in RGB representation superposed onto the shaded relief image of Bosumtwi impact crater. K: 0–2.5 wt% (red channel), Th: 0–9 ppm (green channel), U: 0–4 ppm (blue channel). Dotted lines correspond to the rough areas identified in Fig. 6 and are associated with relatively higher potassium concentrations. Map projection as in Fig. 4.(Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

absent within the ring moat. The presence of a thin (1–2 m) layer of clays within the ring moat was also reported by Boamah and Koeberl (2006) from the shallow drill hole sites BH1 and BH3 (see fig. 3 in Boamah and Koeberl [2006] and Figs. 4, 6, and 7 for the locations of BH1 and BH3).

The association of K-rich areas with rougher topography and outcrops of metasediments indicates that K-rich areas are currently eroded, bringing new fresh rock to surface. K-poor areas correspond to saprock or lateritic cover, where the K originally present in the metasediment has been leached by fluids. It is therefore possible to conclude that the outer ridge is currently experiencing erosion, whereas limited erosion within the ring moat favors the development of a recent lateritic cover. The ring moat is therefore not the result of preferential erosion of ejecta, but it is a

topographic feature that was formed during ejecta emplacement.

Impact breccia, suevite, and unclassified breccia have been observed at several locations on the annular plateau (Koeberl and Reimold 2005; see their geological map for the locations of breccias). Breccia has also been observed at several outcrops of metasediments in the area of rough topography along the outer part of the ridge. Boamah and Koeberl (2002a, 2002b) also reported 8 m thick and 13 m thick suevite deposits from analysis of cores from shallow boreholes at the BH1 and BH3 sites (see Figs. 4, 6, and 7 for these locations). These suevites are composed of melt bodies and crystalline and metasedimentary rocks (granite, greywacke, phyllite, shale, schist, and possibly slate) fragments. Boamah and Koeberl (2006) reported shock metamorphic effects such as PDFs, diaplectic quartz,
<figure><img src="image_9.png" /><figcaption>Fig. 8. Radial profiles comparing concentrations of K (wt%) with elevation. Locations of the profiles are indicated in Fig. 7. The ring moat corresponds to low surface concentrations of K, whereas the rim and outward slope of the ridge are associated with higher concentrations of K. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

feldspar glasses, lechatelierite, ballen quartz, and biotite with kink bands in the clasts. These occurrences are regarded as remnants of an ejecta facies (Boamah and Koeberl 2002a, 2002b).

Preferential erosion at the rims and outer ridge, not within the moat, and evidence that the plateau is composed of several tens of meter of ejecta material (presence of suevite, impact breccias) support that the outer plateau is an ejecta layer, whereas the ring moat

and the topographic ridge were present immediately following impact and ejecta emplacement. Considering that the rim and outer ridge are currently eroded, these topographic features may have been even more pronounced 1 million years ago. The presence of an outer ridge at the edge of the ejecta layer therefore confirms that the ejecta were not emplaced by ballistic processes exclusively, but did involve surface flow. This preservation of ejecta deposits over 1 Ma appears to
<figure><img src="image_10.png" /><figcaption>Fig. 9. Illustration of the lateritic cover observed outside of the outer ring of Bosumtwi. a) Outcrop BO20 (6°30′39.00″N, 1°31′27.37W), pen 10 cm for scale. b) Outcrop BO29 (6°29′10.53″N, 1°31′34.07″W), portable gamma-ray spectrometer RS-230-BGO (26 cm long) for scale. The locations of the two outcrops are reported in Figs. 4, 6, and 7. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

be consistent with the relatively low erosion rate (typically 5–15 m Ma⁻¹ according to Beauvais and Chardon 2013) generally encountered in this part of the West African Craton. Accordingly, we conclude that the Bosumtwi impact structure is a good terrestrial analog for Martian rampart craters and rampart craters on other planets, which is the focus of the next section.

DISCUSSION—THE VALUE OF BOSUMTWI TO UNDERSTAND THE FORMATION OF RAMPART CRATERS

The morphologies of rampart craters, on Mars and more generally in the solar system, are diverse. On Mars, this diversity has led Martian scientists to define a standard classification scheme (Barlow et al. 2000). The Bosumtwi impact crater would be classified as either a single-layer ejecta or double-layer ejecta crater, depending on the possible existence of another thinner outer layer of ejecta that now has been entirely eroded. A possible indication of this layer is suggested by a few exposures of (undifferentiated) breccia (Koeberl and Reimold 2005) and the occurrence of a smooth area

beyond the outer ridge. A topographic feature at about 15–20 km from the center of the crater was also mentioned by Wagner et al. (2002).

Bosumtwi is, at present, the second possible analog for Martian rampart craters on Earth. Sturm et al. (2013) argued that the Ries ejecta blanket involves a continuous rampart structure at 1.45–2.12 crater radii from the crater center, and therefore resembled the ejecta around some of the Martian impact craters named rampart craters. Those authors concluded that the ejecta distribution and thickness at the Ries impact structure is best explained by the presence of fluids mixed with solid ejecta particles during the emplacement process. Note that the Ries impact crater could also be classified with a double-layer ejecta blanket, based on the mapping of the ejecta distribution and similarity with Steinheim crater on Mars (Sturm et al. 2013).

The runout distance of ejecta on Mars is often interpreted as an indication of the degree of fluidization by subsurface volatiles. Ejecta mobility (EM), defined as the ratio of the radius of the ejecta layer divided by the radius of the crater, is thought to reflect the abundance of subsurface volatiles. We note that the EM for
<figure><img src="image_11.png" /></figure>

Fig. 10. Exposed outcrops of metasediment at the top of the crater rim. Outcrop BO33 (6°27′52.57″N, 1°26′14.45″W), which are generally associated with elevated concentrations of K. (Color figure can be viewed at wileyonlinelibrary.com.)

<figure><img src="image_12.png" /></figure>

Fig. 11. Weathered metasediment (saprock) exposed on surface, outcrop B40 (6°31′37.03″N, 1°20′59′16″W). The location of this outcrop is indicated in Figs. 4, 6, and 7. (Color figure can be viewed at wileyonlinelibrary.com.)

Bosumtwi is 1.7. This value falls within the typical range of EM values for Martian rampart craters (Cave 1993).

The respective role of subsurface versus atmospheric volatiles to explain the diversity of rampart morphologies remains debated. Rampart crater morphologies have been reported on Venus (Barnouin-Jha and Schultz 1998), which has a dense atmosphere

but does not have subsurface volatiles. They have been also reported for icy bodies of the solar system, including Ganymede (Boyce et al. 2010), Europa, Dione, Tethys, and Charon (Robbins et al. 2018). In the case of Charon, the low resolution of images did not allow to assure the presence of the rampart, and the acronym ATE for “abrupt termini ejecta” was coined for this reason.

In contrast to the case of Venus, the subsurfaces of these bodies are volatile-rich, but surface atmospheric pressure is very low. Several decades of observation and analysis of rampart craters in these different contexts did not settle the debate on the respective role of subsurface versus atmospheric volatiles in controlling the morphology of ejecta deposits. The existence of a well-preserved rampart crater on Earth offers therefore a new perspective to understand their formation. Though Earth has a dense atmosphere and subsurface volatiles were likely present at the time of formation of Bosumtwi, the preserved ejecta layer of Bosumtwi offers an avenue for future investigation to better understand its emplacement. This could, for example, include studies of clast distributions and sorting processes (Baratoux et al. 2005), as well as analysis of water content in the ejecta. If subsurface volatiles play a dominant role in the formation of ejecta ramparts, the Bosumtwi impact crater may be also useful to better interpret the Martian ejecta in terms of distribution of subsurface volatiles, including especially water-ice, which represents a critical resource for human exploration of Mars.

## CONCLUSION

We have provided evidence that the circular plateau composed of a ring moat and outer ridge at Bosumtwi impact crater is the partially eroded ejecta layer of the crater. Moreover, combining topographic analysis with an airborne survey and field observations, we conclude that the ring moat and outer ridge were present immediately after the impact event and are features formed during ejecta emplacement. The presence of the moat and ridge is a diagnostic feature of ejecta deposits and indicates that ejecta were not emplaced by purely ballistic processes. The ejecta emplacement mechanism involved surface flow and ejecta fluidization, as a consequence of either subsurface water or volatiles present in the atmosphere. Together with evidence for rampart ejecta at the Ries impact structure, this study suggests that rampart ejecta may be as common for large terrestrial impact craters as they are on Mars. Further analyses are needed to determine if the few other large and recent impact structures (Bigach, El'gytgyn, Karla, Zhamanshin) were also associated
with rampart ejecta at the time of their formation. Impact ejecta at Bosumtwi are well preserved in comparison with other terrestrial structures of similar sizes, as a consequence of the relatively low denudation rates in West Africa. The ejecta at Bosumtwi may offer opportunities to elucidate the role of subsurface versus atmospheric volatiles for ejecta fluidization. This presence of a well-preserved ejecta layer raises further the scientific value of the Bosumtwi impact crater in Ghana.

**Acknowledgments**—This research was funded by INSU (National Program of Planetary Sciences, France) and by the West African Exploration Initiative (WAXI). We acknowledge AMIRA International and the industry sponsors for their support of the WAXI project (P934A). The Barringer Family and the “Service de Coopération et d’Action Culturelle” at the French embassy in Senegal are acknowledged for financial support awarded to C.A.B. Niang. D. Baratoux’s guest research position at the Institut Fondamental d’Afrique Noire Cheikh Anta Diop was supported by the French National Institute for Sustainable Development. The geological map of Bosumtwi used in this manuscript is available at http://opac.geologie.ac.at/wwwopacx/wwwopac.ashx?command= getcontent&server=images&value=JB1451_031_A.pdf. This research is also part of a continent-wide effort to promote Meteoritics and Planetary Sciences, namely the Africa Initiative for Planetary and Space Sciences (http://africapss.org), which is supported by the Geochemical Society and the European Association of Geochemistry. We are grateful to Dr. D. Yamazaki for providing access to the denoised SRTM data.

**Editorial Handling**—Dr. Jeffrey Plescia

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