Multiscale Geoelectrical Properties of the Rochechouart Impact Structure, France

Abstract

Terrestrial mid-size (apparent diameter D = 10-60 km) complex impact structures are usually heavily eroded, and often buried below a post-impact sediment cover or filled by a lake, thus precluding a clear estimate of the crater size (Melosh, 1989). The presence of a central structural uplift is systematic and characterizes this category of craters (Osinski & Pierazzo, 2013). Osinski et al. (2008) mentioned that massive impact melt rock (IMR) formations are usually observed for crystalline targets while for sedimentary targets, the melt appears more scattered in breccia. The distinction between clast-rich/clast-poor IMR, impact melt-rich/ melt-poor breccia, fractured/brecciated basement, and unaffected basement is however difficult for these highly eroded (or buried/underwater/covered by vegetation) structures. Thus the only way to distinguish between these types of units requires drilling and geophysics where impacts are buried. In those cases, geophysics provides a means of investigating the lateral and vertical extent of the impactite formations and of the brecciation/fracturing in the basement. Still numerical models of the sources of geophysical anomalies are limited by non-uniqueness and require additional geological data from drillings or trenches. This is particularly true for models built using potential-field data, which are the most widely used geophysical data for investigating impact structures, since the associated signatures are usually significant (e.g., a circular low gravity anomaly). Other geophysical methods (e.g., seismic reflection and refraction) suffer less from this issue. However, the few examples of electrical investigations over impact structures also reveal the necessity for additional constraints, from drilling to other geophysical data (see Henkel, 1992;Pilkington & Grieve, 1992), in order to reduce the ambiguity in the geological interpretations of the electrical contrasts.