An integrated geophysical study of the western part of the Rochechouart-Chassenon impact structure, Charente, France

Abstract

A multi-method geophysical survey was carried out with the aim of reassessing the structure of the Rochechouart-Chassenon astrobleme and the geometry of the impact-related formations, referred to as fallout breccias, in the vicinity of Chassenon. Gravity data extracted from the BRGM InfoTerre database were reprocessed and revealed both the shape of the astrobleme and the presence of a small negative gravity anomaly close to Chassenon, due to the low density of impact-related rocks; a 2D gravity model across the fallout deposits was provided. Geological settings, well-logging and geophysical soundings (direct current (DC), time-domain electromagnetic (TDEM) and frequencydomain electromagnetic (FDEM)) were carried out on the fallout breccias at Chassenon. The ID resistivity models based on TDEM and DC soundings, resulting from a mutually constrained inversion and coupled with ID models of PDEM data, were constrained by borehole data and provided 2D resistivity models across the fallout deposits of Chassenon. The combination of these methods allows the description of both the impact-related rocks geometry and the global structure of fallout breccia remains at Chassenon. The study also shows that the fallout breccia has a complex structure, consisting of thin layers of lithic breccia and suevite, the total thickness of which is larger at Chassenon than previously estimated by means of geological analysis. © 2010 European Association of Geoscientists & Engineers.