THE SIZE/SHAPE OF THE ROCHECHOUART IMPACT CRATER (FRANCE)?

Rochechouart is a deeply eroded late Triassic impact crater. It exclusively affects granites and gneisses of the Hercynian basement of the Massif Central (France). The crater has lost any impact related topography. A variety of indirect evidences have been used to estimate its initial size and shape: E1-Stratigraphy/petrography complemented by recent geological data from the 2017-2018 drilling campaign realized across the central part of the structure, E2-Gravimety (late 70' data, reprocessed in the 80' then in 2015),- E3-Shatter cones distribution, E4-Electric tomography (ongoing studies by CIRIR members), E5-Detailed sedimentology study of the deposit in the SC2 drilling near Chassenon (interpreted as a resurge deposit: ongoing study by CIRIR members).

E5 calls for a small-size crater (12 km) as deduced from the comparison with other impact resurge deposits at other localities. It requires a central peak, typical of complex crater of that size in crystalline target. Rochechouart is a rare if not a unique case where the 3D topography of the floor of an impact crater is widely accessible and easy to map. Allochthonous materials form a very thin (0-100 m) "continuous" stratigraphic unit entailed by the river drainage, exposing the target underneath. There is no evidence of a raise of the contact. The variation of elevation of the bottom of the deposit is +/- 50 m over the entire 10-12 km zone and compares to that of the topography. The remains of the impact deposits thus occupy the bottom of a flat floored circular depression with nowadays an inverted topography.

E2 gives a diameter estimate up to 30-35 km for the initial crater and agrees with E1 to locate the impact point near Valette, which also matches the geometric centre of the preserved breccia deposit. E3 gives a similar size estimates but sees the impact point further South. These estimates still imply a central peak, as illustrated by even larger impact structures in crystalline targets (such as Charlevoix (Canada) for instance). E1 and E4 agree with a collapsed central peak resulting in either a peak ring crater (50-80 km) or a transitional shape between peak ring and central peak (~50 km?).

The large discrepancies between the various estimates lead to the conclusion that we miss something important. More work is thus needed at Rochechouart and more generally at other large terrestrial impact craters.