Lunar and Planetary Science XLVIII (2017) 2583.pdf

THE BOSUMTWI , A TERRESTRIAL RAMPART CRATER. G. Wulf1 and T. Kenkmann1, 1Institute of Earth and Environmental Sciences -Geology-, Albert-Ludwigs-University Freiburg, Ger- many, [email protected].

Introduction: The Bosumtwi impact crater in resolution of 1 arc-second (~30 m at the equator) were Ghana (06°30`N, 01°25`W) is one of the youngest and used as base data to derive catchment areas and drain- best preserved terrestrial impact craters [1, 2]. The age networks in the surrounding of Bosumtwi crater 1.07 Ma old impact structure has a pronounced crater (Fig. 1a). For morphometric comparisons, we deter- rim with a diameter of 10.5 km [3]. The circular struc- mined the mean elevation of the topography at defined ture is somewhat distorted in the SE sector by the radial distances from crater center (bins of 200 m) in northern flank of the NE–SW trending Obuom Moun- order to construct an averaged radial elevation profile tain Range [4] (Fig. 1a). The impact structure forms a (Fig. 1b). In the process, the southern margin was hydrologically closed basin that is almost completely cropped and not incorporated into the analysis because filled by the 8.5 km diameter Lake Bosumtwi. The this area is strongly affected by the northern flank of impact crater was intensely studied in the framework the NE–SW trending Obuom Mountain Range (Fig of various drillings campaigns, e.g., the International 1a). Continental Scientific Drilling Program (ICDP) drill- ing in 2004 [5, 6]. Suevites (polymict, glass-bearing breccias) and polymict clastic impact breccias were found in the boreholes as well as in the northern and southern parts within the crater structure. Although chemical weathering is intense in the tropical rain for- est environment and led to the formation of locally thick lateritic soils, a lot of breccias associated with the formation of the crater are still preserved outside of the crater rim (see review by [2]). Moreover, polymict and monomict clastic breccias were reported from numer- ous places around the impact crater whose impact origin was not unambiguously confirmed [2]. A strik- ing feature of the Bosumtwi impact structure is a shal- low, near-circular, depression at 7-8.5 km from the crater center directly beyond the crater rim, followed by a shallow outer topographic ring feature at 18-20 Fig.1: SRTM elevation data and derived averaged radial km radial distance from the impact center [7, 8] (Fig. elevation profile of Bosumtwi crater (A/B) show a circular 1b). This feature is visible in radar satellite images as depression followed by an elevated ring beyond the crater well as in aero-radiometry data, indicating lithological rim of Bosumtwi crater; CTX DTM and derived averaged as well as topographic control [4, 8, 9]. It was suggest- radial elevation profile of the Martian DLE crater Steinheim ed that preferential removal of ejecta within the area (C/D) [data from 11] show similar results including moat and just outside of the crater rim could be the reason for rampart. this shallow depression [8] or original depositional patterns as well as impact-induced concentric fractur- In addition, we used the principal component anal- ing could also be involved [4]. ysis (PCA) on the base of Landsat and ASTER data to Here we present preliminary results showing that emphasize variations in the multispectral data and thus the morphological features (circular depression and bring out strong patterns in the dataset (Fig. 2b). In topographical ring) beyond the crater rim of the further steps, we compared the results with a Martian Bosumtwi crater are the remnants of an ejecta rampart rampart crater of similar size. The Martian crater and thus, that the Bosumtwi crater is a rampart crater. Steinheim (190.65°E 54.57°N)[11] is a young 11.2 km Methods: The Bosumtwi crater in Ghana was ana- diameter double-layered ejecta (DLE) crater in Arca- lyzed in detail on the base of remote sensing data. A dia Planitia and has an excellent coverage of high- GIS environment was implemented for geospatial resolution image data. We used the Ames Stereo Pipe- analyses of digital elevation models (DEMs) and mul- line to build a high-resolution digital elevation model tispectral analyses. Digital elevation data from the (DEM) based upon CTX imagery and MOLA ( Shuttle Radar Topography Mission (SRTM) with a Lunar and Planetary Science XLVIII (2017) 2583.pdf

Orbiter Laser Altimeter) data for further detailed anal- multispectral data of this area provide no clear signa- yses of the ejecta morphology of Steinheim crater [10], ture for possible ejecta deposits due to the dense rain- including a radial elevation profile and the determina- forest cover, a weak halo-like signal surrounds the tion of a hypothetic drainage pattern and catchment impact crater possibly indicating a similar composition areas (Fig. 1c, d). of the moat and rampart area (Fig. 2b). Following this approach, it is conspicuous that the drainage network around Bosumtwi crater is showing a circular pattern in the slightly depressed annular zone (Fig. 3a). As- suming Bosumtwi crater as the result of a lunar like , the thickness of the ejecta blanket would decrease with increasing radial distance to crater center as the result of ballistic sedimentation and erosion. Under such circumstances, a radial drainage pattern would be expected in contrast to the actual concentric pattern. As a thought experiment, we have generated a hypothetical drainage network for the Martian DLE Fig.2: A) Mapped breccias in the surrounding of Bosumtwi crater Steinheim. Interestingly, the drainage pattern of crater (data from [2]: red = suevites (polymict, glass-bearing Steinheim crater shows a lot of similarities to breccias); white circles = polymict clastic impact breccias; Bosumtwi including a local watershed along the crest- white triangles = polymict clastic breccias (genetically un- line of the rampart and a concentric discharge pattern classified, likely impact-related); black circles = polymict in the moat area (Fig. 3b). and monomict clastic breccias (unspecified, possibly impact- Conclusions and Outlook: The morphological related). B) False colour RGB image of the Principal com- features (circular depression and topographical ring) ponent analysis (PCA) on the base of ASTER VNIR data. beyond the crater rim of Bosumtwi crater as well as

the ejecta distribution and drainage pattern show many similarities to Martian rampart craters. Therefore, we suggest that Bosumtwi crater shows the eroded rem- nants of an ejecta rampart, possibly similar to the Ries crater in Germany [12], and thus, is a terrestrial ram- part crater. In future works, this assumption will be proven in more detail by using TanDEM-X imagery of higher resolution and further multispectral data anal- yses. References: [1] Scholz, A.C. et al. (2002) Geology, vol. 30, pp. 939–942. [2] Koeberl, C. and Reimold, W.U. Fig.3: Derived drainage pattern and accordant catchment (2005) Jahrbuch der Geologischen Bundesanstalt, vol. 145, areas for Bosumtwi crater (A) and the Martian Steinheim pp. 31–70. [3] Koeberl, C. et al. (1997) Geochim. crater (B). Cosmochim. Acta, vol. 61, pp. 1745–1772. [4] Reimold,

W.U. and Koeberl, C. (2014) Journal of African Earth Sci- Results and Discussion: The SRTM elevation da- ences, vol. 93, pp. 57-175. [5] Turner, B.F. et al. (1996) ta and the derived averaged radial elevation profile Limnol. Oceanogr., vol. 41, pp. 1415–1424. [6] Koeberl, C. show a circular depression followed by an elevated et al. (2007) Meteoritics & Planetary Science, vol. 42, pp. ring beyond the crater rim of Bosumtwi crater (Fig. 1a, 477–896. [7] , W.B. et al. (1981) Ghana. Geol. Soc. b). The morphological characteristics of this topo- Am. Bull., vol. 92, pp. 342–349. [8] Wagner, R. et al. (2002) graphic pattern show striking similarities with regard In Meteorite Impacts in Precambrian Shields. Impact Stu- to position, dimension, and shape to those of Martian dies, vol. 2: Plado, J., Pesonen, L.J. (Eds.), Springer, Berlin, DLE craters, which typically possess depressions Heidelberg, pp. 189–210. [9] Pesonen, L.J. et al (2003) (called moats) and subsequent broad, elevated ridges Yearbook of the Austrian Geological Survey, Vienna, vol. (called ramparts) [11] (Fig. 1c, d). In the case of Mar- 143, pp. 581–604. [10] Moratto Z.M. et al. (2010) 41st Lunar tian craters, this morphological trend is due to the dis- and Planetary Science Conference, abstract #2364. [11] tribution of the ejecta blanket. Indeed, Bosumtwi also Wulf G. and Kenkmann T. (2015). Meteoritics & Planetary shows a lot of evidence for ejecta deposits within this Science, vol. 50, pp. 173–203. [12] Sturm S. et al. (2013) area including a multitude of impact-related and possi- Geology, vol. 41(5), pp. 531-534. bly impact-related breccias (Fig. 2a). Although the