Aouelloul Impact Crater, Mauritania: New Structural and Lithological Data
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Large Meteorite Impacts VI 2019 (LPI Contrib. No. 2136) 5093.pdf AOUELLOUL IMPACT CRATER, MAURITANIA: NEW STRUCTURAL AND LITHOLOGICAL DATA. E. cheikh Ould Mohamed Navee1, H. Chennaoui Aoudjehane1, D. Baratoux2, L. Ferrière3, and M. S. Ould Sabar4, 1GAIA, Laboratory, Hassan II University of Casablanca, Faculty of Sciences Ain Chock Casablanca, Morocco ([email protected]), 2Geosciences Environnement Toulouse, University of Toulouse, Research Institute for Development & CNRS, 14, Avenue Edouard Belin, 31400 Toulouse, France, 3Natural History Museum, Burgring 7, A-1010 Vienna, Austria, 4Department of Geology, Faculty of Science and Technology University of Nouakchott El-Aasriya, Nouakchott, Mauritania. Introduction: The Aouelloul impact crater is lo- Bing Maps aerial image of the Aouelloul crater. The cated in Mauritania, in the Adrar region, about 50 km locations of impact glass fragments collected at several SE from the city of Atar. It was first discovered from locations around the crater are also reported, allowing the air by A. Pourquié in 1938 and visited by him on us to update the spatial distribution of the glass, which the same year and again in 1939. The crater was first was limited to a very small sector at the SE of the described in the scientific literature by Monod and crater on the map by [1]. All information were reported Pourquié in 1951 [1]. However, it was already known with the ArcGis software on the aerial image. for a very long time already from the local people who call it "Hofrath Aouelloul" (i.e., the “hole of Aouel- loul”) [1]. The crater, with a well-developed rim, is almost 400 m in diameter. Fission track and K-Ar da- ting of impact glasses found around the crater yielded an age for the crater of 3.25 ± 0.50 Ma [2] and 3.1 ± 0.3 Ma [3], respectively. It is formed in sandstones from the Oujeft and Zli Formations, both Ordovician in age [1]. It was confirmed to be of impact origin based on the presence of an extraterrestrial component in the glass fragments found around the crater [4]. So far only a few quartz grains with “possible remnants of planar deformation features (PDFs)” were reported by [4] from fractured sandstone samples collected from the crater rim. We report here on new structural and litho- logical data acquired during a 2019’s field campaign. Field campaign, method and data: One of the Fig. 1. Typical outcrop of the sandstone/quartzite be- main source of information about the Aouelloul impact longing to either the Zli or the Oujeft Formations as crater comes from field observations published in [1; seen on the rim of the crater. only available in French language]. In order to update this work and to better understand the structural defor- Lithological mapping of the Aouelloul impact mations associated with this small impact crater, a crater: Most of the interior of the impact crater is fieldwork campaign was organized at Aouelloul in filled with recent (quaternary) sand deposits. Solid February 2019. All co-authors of this abstract went in rocks outcrops are found almost exclusively in the in- the field. The main objectives were to collect structural ner flanks of the crater rim, mostly restricted to its measurements and a number of samples from outcrops north and west parts. Macroscopic examination of (Fig. 1) as well as to collect information on the Aouel- these outcrops revealed that they consist of more or loul impact glass distribution. A number of impact less weathered sandstone and quartzite (Fig. 1). The glass samples were also collected and some were ac- distinction between both lithologies was difficult in the quired from local persons (Fig. 2). All the collected field and will requires petrographic examinations of information is reported on a new lithostructural map of thin sections of the collected samples to be completed. the crater and its surroundings (Fig. 3). We spent a Structural mapping of the Aouelloul impact total of three days at and around the impact crater. crater: The rim of the impact crater is characterized by More than a dozen of rock samples were collected more or less disturbed meter to pluri-meters rock along the rim and structural measurements at 16 out- “blocks” showing fracturing and dipping toward either crops (i.e., strike and dip of sedimentary layers, and/or the interior or the exterior of the crater (Figs. 1 & 3). orientations of fractures) were obtained. Features al- Both radial and concentric fracturing were observed. ready reported in [1] were checked and compared with We were able to measure only three radial and six con- our data taking into account that at the time they have centric fractures in total. prepared their map, GPS was not available to collect Aouelloul impact glass: A number of Aouelloul accurate coordinates. Our field data are overlaid on a impact glass samples were collected. They range in Large Meteorite Impacts VI 2019 (LPI Contrib. No. 2136) 5093.pdf size from less than a centimeter up to 9 cm for the larg- terior of the crater. This asymmetric distribution may est one. The Aouelloul impact glass is mainly dark gray be the result of an oblique impact (if original distribu- in color, but some dark to light green, to yellowish or tion is preserved), or just the result of a difference in even rare beige samples were also found (Fig. 2). The preservation (i.e., no impact glass samples were found location of some of the collected samples is reported in the north and west parts). on the map (Fig. 3) but some local transport after depo- Conclusion and perspectives: The preliminary sition cannot be excluded. lithostructural map of the Aouelloul impact crater pre- sented here shows for the first time the structural meas- urements and accurate geographic coordinates of all components of the Aouelloul impact crater, including the impact glass distribution. Petrographic investiga- tions of the collected samples are currently in progress and results will be presented at the conference. We hope to find shocked quartz (i.e., with PDFs) and to characterize their orientations. Acknowledgements: This work is part of the Afri- can Initiative for Planetary and Space Science (http://africapss.org) and the ATTARIK Foundation Fig.2. Macrophotograph of a selection of impact glass (www.attarikfoundation.org). It is supported by the fragments collected by L.F. around the Aouelloul crater Barringer Family Fund, the Mauritanian Office of Geo- (scale bar is 1 cm). logical Research, the University of Nouakchott Al Asriya, the Uranoscope of France, and the French Insti- Discussion: All the collected information, structur- tute of Research for Sustainable Development (IRD). al measurements, rocks and impact glass distribution References: [1] Monod Th. and Pourquié A. were reported on an aerial image of the Aouelloul im- (1951) Bulletin de l'IFAN, 13, 293–311. [2] Storzer D. pact crater (Fig. 3). Fracturing seems to be mostly con- and Wagner G. A. (1977) Meteoritics, 12, 368–369. centric. Impact glass fragments are mainly found in two [3] Fudali R. F. and Cressy P. J. (1976) Earth and areas, in the eastern and in the southern part of the ex Planetary Science Letters, 30, 262–268. [4] Koeberl C. et al. (1998) Meteoritics & Planet. Sci., 33, 513–517. Fig. 3. Preliminary lithostructural map of the Aouelloul impact crater. Data are overlaid on a Bing Maps aerial image .