Exploration of the hypothetical impact ejecta layer around the Maâdna structure (Talemzane, Algeria) using georadar technique

Abstract

The Maâdna structure (Talemzane, Algeria) is located approximately 400 km south of Algiers and is formed within Upper-Cretaceous to Eocene limestones from the Saharan Platform. It is a bowl-shaped crater roughly 1.7 km in diameter. The present-day crater rim is 60 m higher than the bottom of the depression, and is raised slightly compared to the surrounding terrain. So far, several aspects of Maâdna structure cratering process remain poorly known. Recent study, geological and geophysical investigation, do not support an impact origin for this structure but suggest an alternative dissolved diapir with inverted relief. At present, geophysical studies revealed many interesting features of known terrestrial impact craters. In particular, observed signatures yielded useful information on the impact-induced physical changes in targeted rocks. These observations motivated our investigation of Maâdna crater, which uses several different geophysical surveys with particular emphasis on the GPR technique. This later is used as a valuable tool for near surface explorations. Although GPR technique has a limited use in case of impact structures, it is used in this study to discriminate between electromagnetic signatures of supposed ejectas, which were widely used to describe Maâdna structure origin. Typical GPR antennas were used to test a survey protocol in several situations during the GPR field deployment stage. In order to characterize the target thin and shallower layer, most of the common profiles were acquired using a 500 MHz shielded antenna. Otherwise, to ensure maximum waves penetration depths, “NS & EW” profiles were also acquired using a lowest frequency 30 MHz unshielded rough terrain antenna (RTA) system with a flexible “snake-like” design. Moreover, the ensued GPR characterization is then used to constrain occurred cratering process. Our GPR survey results helped us to improve our knowledge of the structure subsurface features expression. A suitable post-acquisition processing and analyses of the collected data lead us to reject the existence of any electromagnetic signature that may be attributed to the presence of any atypical materials, such as melted iron-rich material or those with distinctive physico-chemical properties, which induces electromagnetic signal modification or dissipation. Besides, our different scans could help truthful geological interpretation linked to Maâdna structure. For example, scans operated on the prevailing structural unit surrounding the Maâdna crater show clearly the most documented structural features at this crater, such as bedrocks fracturation, locally collapses, overturned and/or vertical movement of strata, etc. Specifically, the topographically corrected scans are presently interpretative against the alternative scenario of the cratering process at Maâdna, since our various analyses do not support any meteoritic evidence. As a result, valuable insights into the hypothetic diapiric piercement have been then gained from this analysis. This leads us to conclude that GPR data can be used to build another strong argument in favor of the diapiric origin at Maâdna crater.