The Rochechouart crater: Shock zoning study

Rochechouart crater (France) occurs in crystalline rocks of the northwestern French Massif Central. The crater is deeply eroded and the present ground level is exactly (±50m) tangent to the crater floor. No morphologic evidence of the crater has been preserved. A complete range of shock effects is known on the scale of rocks and minerals, which permitted a study of shock zoning. Shock level was determined in thin section from petrographic analysis of each specimen. A systematic sampling was done on all the structure. Shock zones were determined at the same time in the fall-back unit and in the target. Correlations between rock types or shock level in allochthonous and autochthonous materials were observed. They imply restrictions for the late excavation stage: part of the material lying above the crater floor limit was never ejected, but only mixed with small relative displacement. Shock level is relatively higher in allochthonous breccias than in the target. The distribution of shock effects is very complicated at all scales, even at the scale of the whole structure. However, shock level is relatively high around the center of the structure defined from the breccia geometry. The most probable impact point is located about 4 km west of Rochechouart. The breccia unit is extremely thin (less than 60 m). The crater floor is extremely flat; its elevation does not vary more than±50m over the whole structure (about 300 km²). The lack of circular symmetry in particular in the distribution of impact melts could suggest a pronounced anisotropic structure of the target and/or an inclinated trajectory of the projectile. The original crater size is most probably between 20 and 25 km in diameter, determined from the actual extent of differents rock types or shock effects. A rapid post-crater readjustment is proposed to explain the flat floor. There was no important vertical displacement since the flat shape of the crater floor was attained.