Geophysical and magneto‐structural study of the Maâdna structure (Talemzane, Algeria): Insights on its age and origin

Abstract

International audienceThe Maâdna structure is located approximately 400 km south of Algiers (33°19' N, 4°19' E) and emplaced in Upper-Cretaceous to Eocene limestones. Although accepted as an impact crater on the basis of alleged observations of shock-diagnostic features such as planar deformation features (PDFs) in quartz grains, previous works were limited and further studies are desirable to ascertain the structure formation process and its age. For this purpose, the crater was investigated using a multidisciplinary approach including field observations, detailed cartography of the different geological and structural units, geophysical surveys, anisotropy of magnetic susceptibility, paleomagnetism, and petrography of the collected samples. We found that the magnetic and gravimetric profiles highlight a succession of positive and negative anomalies, ones that might indicate the occurrence of a causative material which is at least in part identical. Geophysical analysis and modeling suggest the presence of this material within the crater at a depth of about 100 m below the surface. Using soil magnetic susceptibility measurements, the shallowest magnetized zone in the central part of the crater is identified as a recently deposited material. Paleomagnetic and rock magnetic experiments combined with petrographic observations show that detrital hematite is the main magnetic carrier although often associated with magnetite. A primary magnetization is inferred from a stable remanence with both normal and reverse directions, carried by these two minerals. Although this is supposed to be a chemical remagnetization, its normal polarity nature is considered to be a Pliocene component, subsequent to the crater formation. The pole falls onto the Miocene-Pliocene part of the African Apparent Polar Wander Path (APWP). Consequently, we estimate then formation of the Ma^adna crater to have occurred during the time period extending from the Late Miocene to the Early Pliocene. Unfortunately, our field and laboratory investigations do not allow us to confirm an impact origin for the crater as neither shatter cones, nor shocked minerals, were found. A dissolved diapir with inverted relief is suggested as an alternative to the impact hypothesis, which can still be considered as plausible. Only a drilling may provide a definite answer

Field gamma ray spectrometry as a diagnostic tool for the assessment of hypothetical melt rocks from the Maâdna structure (Talemzane, Algeria): Implications for the crater formation process

“…They have defined evidence for carbonate melting as effects resulting from shock metamorphism. In contrast, Lamali et al (2016, 2020) pointed out the previous authors’ misinterpretation of an unusual breccia or what appears as black varnish on weathered limestone and/or some type of outcrops (e.g., Pliocene breccias or neoformed concretions), as melting materials. An analogy was already made by Lamali et al (2016, 2020) between these litho‐type zones, within and outside of the crater, and referred to their fieldwork and petrographic analysis of their large number of samples.…”

“…1a). Lamali et al (2016) used topographic profiles constructed from Shuttle Radar Topography Mission (SRTM) data to indicate an eccentricity of about 1.093. These authors identified a simple bowl shape for the structure with a slightly elliptical topographic depression, with long and short axes of 1.75 and 1.6 km in N‐S and E‐W edge dimensions, respectively.…”

“…However, its actual formation mechanism is still controversial. Lamali et al (2016Lamali et al ( , 2020 reported data from detailed fieldwork, structural geological analysis, and geophysical studies that were not conclusive to provide unambiguous supporting evidence for an impact interpretation. They concluded that the Maâdna structure could alternatively be the result of a local diapiric emplacement, although they did not exclude the impact hypothesis.…”

Life on Mars (LoMars): A bibliometric study of current research and future perspectives

“…A piece of evidence comes from the publication survey (i.e., 2000-2015), revealing that only less than 1% of the worldwide output of scientific papers have been published in the (BENKHALDOUN et al 2005;GILLON et al 2017). Algeria is also focusing on meteorites and impact craters with the help of international collaborations (LAMALI et al 2016;SAHOUI et al 2016). Since 2015, the Arabian Geosciences Union have launched a special section on planetary science and astrobiology for PSS scientists across northern Africa and the Middle East.…”

The fourth Arab Impact Cratering and Astrogeology Conference (AICAC IV), April 9–12, 2017, Algiers (Algeria)

“…Emphasis was given on the African impact structures and meteorites, considered with geological and/or geophysical approaches (e.g., Lamali et al. ; Sahoui et al. ).…”