Revisiting the Rochechouart impact structure, France

Abstract

The Rochechouart impact structure, south-central France (45 o 50 0 N, 0 o 46 0 E), is a partly eroded, approximately 200 Myr, complex impact structure. The impactite suite at Rochechouart provides an excellent example of gradational boundaries and transitional lithologies that have been historically difficult to classify with standard impactite nomenclature. Here, we present the first detailed scanning electron microscopy-based description of the Rochechouart impactites integrated with hand-sample and petrographic observations with the goal of understanding the clast-matrix relationships of transitional lithologies. Three main impact-generated hydrothermal alteration assemblages are also recognized: (1) argillic-like, (2) carbonate, and (3) oxide. Our results support the existence of a continuum between clast-rich impact melt rocks and glass-rich clastic breccias (suevites) that must be represented in universal classification schemes. This suite of impactites from the Rochechouart impact structure is used as a test case for a recently published classification scheme based on the nature of the groundmass setting a precedent for classification of impactites with limited to no geological context such as deeply eroded terrestrial impact structures and future sample return missions. The re-evaluation of the melt-bearing Rochechouart impactites questions the currently accepted size of the crater, suggesting a much larger original crater diameter.

A new high‐precision40Ar/39Ar age for the Rochechouart impact structure: At least 5 Ma older than the Triassic–Jurassic boundary

“…The Babaudus material also contains minor amounts of submicron‐sized Fe‐Ti oxides (Sapers et al. ) (Fig. ).…”

“…Rochechouart is a moderate-sized impact structure, with a maximum original diameter of 40-50 km (Lambert 2010;Sapers et al 2014), although postimpact erosion has reduced the exposure to approximately 24 km (Lambert 2010). The structure contains a variety of impactites, ranging from melt-rich to melt-poor material, including impact melt, impact melt-bearing breccia, lithic impact breccia, and ash-sized fallout debris (Lambert 2010;Sapers et al 2014) (Fig. 1).…”

“…As regards to sample selection, 40 Ar/ 39 Ar dating of impact structures can be compromised if the rocks analyzed contain clasts of the target lithologies, which if not fully degassed during crater formation will retain older (i.e., inherited) 40 Ar (e.g., as discussed by Jourdan et al 2007a;Mark et al 2014). To avoid potential complications from target rock clasts, we therefore focused our study on the K-feldspar bearing impact melt at Babaudus, which contains the highest percentage of melt and smallest percentage of preimpact clasts of all of the Rochechouart impactite products (Sapers et al 2014). A sample of fresh Babaudus melt was obtained from the R eserve Naturelle de l'Astrobl eme de Rochechouart-Chassenon.…”

“…The structure contains a variety of impactites, ranging from melt‐rich to melt‐poor material, including impact melt, impact melt‐bearing breccia, lithic impact breccia, and ash‐sized fallout debris (Lambert ; Sapers et al. ) (Fig. ).…”

“…To avoid potential complications from target rock clasts, we therefore focused our study on the K‐feldspar bearing impact melt at Babaudus, which contains the highest percentage of melt and smallest percentage of preimpact clasts of all of the Rochechouart impactite products (Sapers et al. ). A sample of fresh Babaudus melt was obtained from the Réserve Naturelle de l'Astroblème de Rochechouart–Chassenon.…”