¹Axel Wittmann, ²Philippe Lambert
Earth and Planetary Science Letters 674, 119748 Link to Article [https://doi.org/10.1016/j.epsl.2025.119748]
¹Eyring Materials Center, Arizona State University, 1001 S. McAllister Ave., Tempe, AZ 85287-8301, USA
²CIRIR ‒ Centre International de Recherche et de Restitution sur les Impacts et sur Rochechouart, 2-4 Faubourg du Puy du Moulin, Rochechouart 87600, France
Copyright Elsevier

The 205 Ma Rochechouart impact structure in southern France exhibits variable levels of erosion that mask its original diameter for which estimates vary between 10 and 35 km. Exclusively at Chassenon, the size-sorted, “ash-like” impactoclastite deposit occurs as the last preserved material directly produced by the impact. To test whether impactoclastite is indeed a fallback deposit from the impact plume, we studied 18 Rochechouart impactite samples, of which 15 are dike-like intercalations of impactoclastite in suevite from Chassenon. We found 63 impact spherules in 13 samples from Chassenon, down to a drill core depth of 27.65 m. These spherules are impact melt droplets that record suspended flight. Of these spherules, 30 % crystallized Ni-bearing spinel, 11 % contain small NiO particles, and one includes a ∼140 nm Pt-Os-Ru-Ir-Rh-Pd nugget; these are impactor components, confirming formation in close proximity to the point of impact. The exclusive occurrence of impact spherule-bearing impactoclastite associated with suevite at the Chassenon location suggests special formation conditions that we link to the collapse of the Rochechouart central peak, which induced the down-thrusting of the ∼3 km2 “Chassenon slab”. Resulting fissures in suevite were filled with debris that fell back from the ejecta plume one hour to ca. 1 day after the impact. This interpretation negates the deposition of the Chassenon suevite from marine resurgence immediately following the impact. Instead, we invoke a “debris-inhalation” process that injected impactoclastite dikes due to brief vacuum conditions generated in the sub-crater floor during collapse of the Chassenon slab.