PART 1: INTRODUCTION TO THE GEOLOGY OF THE ROCHECHOUART IMPACT STRUCTURE Philippe Lambert Sciences et Applications, 33800 Bordeaux-France
[email protected] Field Guide- Meteoritical Society 2009 12 SUMMARY The 200 Ma old, 24 km diameter Rochechouart impact structure formed in granitic intrusive and metamorphic rocks of Variscan age (400-300 Ma) close to the margin of the Mesozoic sea. Fractured basement and autochthonous breccias form a several–decameter-thick semi-continuous zone over a 18-20 km diameter zone. Impact melt rocks, suevite and polymict lithic breccia spread over a ca 15 km inner zone forming a centro-symmetric deposit inclined 0.6° north. No topographic expression of the central uplift exists. The crater floor is at the same elevation (ca +/- 50 m) over a zone at least 20 km in diameter corresponding to the central part of the original crater. The pre-erosional diameter of the crater is probably larger than previously thought and possibly reached 40-50 km. Despite the patchy character of the remaining crater fill deposits, the structure is much less eroded than it looks, as the sequence of crater fill is complete as exposed near Chassenon. The suevite in Chassenon is capped by an ash-like horizontal deposit of very glass-poor, fine-grained, lithic debris derived from basement rocks. Material with similar grain size and composition is observed in centimeter- to meter-thick multi-layered glass-bearing intercalations (dikes) cutting through the suevite. The integrity of the Chassenon sequence strikingly contrasts with the age and morphology of the structure, implying a rapid and thick sedimentary deposit has covered the crater to protect it from erosion.