No abstract

<p>By measuring changes in radioelement concentrations, gamma-ray spectrometry is increasingly emerging as an efficient geophysical method that allows such changes to be geologically mapped according to lithology and soil type. At Maâdna crater in southern Algeria, this method has been used to monitor any changes in the composition of the target rocks that may be associated with the impact cratering process. For this purpose, several measurements were carried out in situ using a portable field gamma spectrometer. As a result, most predominantly calcareous surface lithologies, exposed on the rim and flanks of the crater, showed a very low emitted radiometric response over the three channels (K, Th, U). However, no more than 90 Cps were counted both inside and outside the crater. Such a rate is indeed expected in sedimentary rocks with low clay content, and this remains valid, as long as other exogenous mineralogical enrichments are excluded. On the other hand, the contoured radioelement concentrations maps, have demonstrated an anomalous enhanced gamma radiation levels of potassium-dominated peaks over the central part of the crater and in the surrounding wadis. Nevertheless, the central potassium anomaly is well correlated with the shallower magnetic one that has been described in previous studies (see e.g. Lamali et al., 2016). Therefore, either near the surrounding wadis or in the central part of this crater, this anomalously high level of radioactivity may be linked to an accumulation of later altered deposits. Consequently, there are no objective criteria to link these results to an impact event occurring at the Maâdna structure, similar to what was done at the Serra da Cangalha crater (Vasconcelos et al., 2012).</p>

<p>Geophysics continues to play a critical role in the future discovery of terrestrial impact structures. While the signatures within these structures may not be unique, the application of geophysics can effectively characterize them, even when they are deeply eroded or completely buried underground. In the case of Maâdna crater (33°19' N, 4°19' E), among new performed geophysical surveys, a GPR technique has been especially used to explore a supposed ejecta layer. However, GPR survey results allowed the confirmation of nonexistence of such as melting materials at Maâdna crater. Nevertheless, our different scans were interpretative against the structural context of the Maâdna structure. Indeed, most of the analyzed profiles allowed us recognizing the typical deformation effects at this structure, which can also generally be encountered at any crater-like structured site. Consequently, in view to this new resulting GPR data, even we do not definitely reject an impact origin, we are still pleading for other caratering scenarios for this structure.</p>