Although no structural, lithological, or sedimentological criteria for a (shallow) marine impact scenario have been found to date, paleoenvironmental considerations suggest that the Rochechouart impact occurred very close to (or even beyond) the latest Triassic shoreline of the continental Massif Central, with the marine Aquitaine Basin and Biscay Rift (westernmost Tethys) to the West (e.g., Curnelle et al. 1982; Ziegler 1990; Smith et al. 1994). This is in accord with earlier suggestions that the Rochechouart impact occurred within a shallow marine continental basin and the detection of elevated amounts of sulfur, phosphorous, and chlorine in the glassy mesostasis of the Champagnac pseudotachylites (Kelley and Spray 1997), which might indicate the influence of seawater.
Recently, Simms (2003, 2007) described a still puzzling approximately 2–4 m thick “seismite” of large extent (>250,000 km2) partially overlain by “tsunamite” in the uppermost Triassic of the British Isles (Cotham Member of the Lilstock Formation, Penarth Group, upper Rhaetian) a few meters below the biostratigraphic Triassic/Jurassic boundary. These peculiar deposits indicative of extensive soft-rock deformation were suggested to be incompatible with endogenic terrestrial mechanisms but consistent with a hitherto unknown high-energy end-Triassic impact event: “The seismite certainly suggests an event that is unique in the Phanerozoic history of Britain and Ireland, and that is consistent with bolide impact,” and “impact of a km-scale asteroid here potentially could produce the observed sedimentological effects across the UK” (Simms 2007; see also Hesselbo et al. 2007 for discussion). Similarly, Mader (1992, p. 306) noted potential tsunami deposits in the Lodève Basin of Southern France (Pégairolles de l’Escalette section, Languedoc-Roussillon; see Courtinat et al. 2003, for stratigraphic column), where an intercalation of “exceptional and quasi-instantaneous character” in the Upper Member of the Rhaetian contains “allochthonous material” and “could be attributed to a tsunami having been triggered by seismic instability in the basin” (Mader 1992). In the Northern Apennines of Italy, Rhaetoliassic sediments of the Grotta Arpaia section (basal Grotta Arpaia beds, Portovenere Limestone Member of the La Spezia Formation) interpreted as basin deposits distal to a carbonate ramp, contain slump deposits with storm layers, the composition of which “implies that they were not due to tectonic activity at the platform margin, but that they originated inside the basin and affected the middle and outer part of the ramp” (Ciarapica 2007). Upper Rhaetian exotic high-energy deposits were also reported from the High Tatra Mountains of the Slovak Western Carpathians (“slumped beds” of the upper Fatra Formation; Michalík et al. 2007).
In agreement with the new 40Ar/39Ar age data and paleogeographic studies, the near-coastal to shallow marine impact of the Rochechouart meteorite (see Fig. 9 for impact site at ∼200 Ma) can be regarded as a potential trigger mechanism for a large tsunamigenic earthquake and the formation of end-Triassic tsunami deposits in the westernmost Tethys domain. The Aquitaine Basin, Biscay Rift, Western Approaches Trough, Bristol Channel, and Burgundy Gate (Fig. 9) (compare paleogeographic maps by Ziegler 1988; Ziegler and Kent 1982; Ziegler et al. 1983; Blakey 2006) represented channel-like sea passages that linked the Rochechouart impact site and the British Isles at the time of impact, maintaining high wave energy. The western Tethyan rift basins represented domains of pronounced subsidence during the Late Triassic (Ziegler and Kent 1982), suggesting a steep relief of the basins and deeper marine conditions. The distance between the impact site and the area of tsunami deposition was about 700–1300 km for the British Isles and notably shorter, about 300 km, for southern France (Pégairolles de l’Escalette). Just as well, the Northern Apennine and Western Carpathian domains were at paleodistances of ≤2000 km in the direction of the Neotethys Sea. Tsunamis, large sea waves caused by tectonic activity (earthquakes and faulting), submarine landslides, and volcanic eruptions, are known to produce distinct sedimentary features of high-energetic and complex erosion and deposition in the coastal run-up areas, commonly characterized by erosive fining-upward sequences (e.g., Bondevik et al. 1997; Dawson and Shi 2000; van den Bergh et al. 2003; Cantalamessa and Di Celma 2005); however, the destructive properties and specific sedimentological features produced by tsunamis strongly depend on the location of the tsunami source, the regional geologic and topographic conditions, as well as on water depth and sea floor geometry (e.g., Dypvik and Jansa 2003; Satake 2007), and are still not fully understood (e.g., Bondevik et al. 1997; Gersonde et al. 2002). During the last decades, tsunami deposits formed by impact-triggered tsunamis have been recognized, e.g., those caused by the Chicxulub impact at the Cretaceous/Paleogene boundary (e.g., tsunamites in Texas and Mexico; Bourgeois et al. 1988; Smit et al. 1996), or in the Archean Hamersley Basin of Western Australia (Hassler et al. 2000). Just as well, tsunamites are associated with the Devonian Alamo impact, Nevada and Utah, USA (e.g., Warme and Kuehner 1998), the Devonian Kaluga impact, Russia (Dypvik et al. 2004), and the Cretaceous Tookoonooka impact, central Australia (Bron 2009). Numerical modeling suggested that the Latest Jurassic marine impact that created the approximately 40 km Mjølnir impact structure in the Barents Sea off Norway also triggered a large tsunami (Shuvalov et al. 2002; Glimsdal et al. 2007). A summary of impact-generated tsunamis is given by Ward and Asphaug (1999), Dypvik and Jansa (2003), Dypvik et al. (2004), and Goto (2008). The Rochechouart impact energy exceeded the energy of the largest man-witnessed terrestrial earthquake (Valdivia, Chile, May 22, 1960; Richter scale magnitude 9.5 and a released energy of ∼1.12 × 1019 J), capable of producing major seismic waves in the Earth’s crust and tsunami waves in the sea. An impact energy of approximately 1.3 × 1021 J (equivalent to an earthquake of Richter scale magnitude 10.8–10.9) would have been associated with the impact of an iron meteorite approximately 1 km in diameter into largely crystalline rocks, at a typical impact velocity of 25 km s−1 and an impact angle of 45° (Marcus et al. 2004; Collins et al. 2005); these values are conservative estimates for the current (minimum) 20–25 km diameter of the eroded Rochechouart impact structure. Assuming a larger, approximately 2 km iron projectile and an original diameter of the Rochechouart impact structure of approximately 40–50 km (Lambert 2010), given the same impact angle and velocity, an impact energy of approximately 1.05 × 1022 J (corresponding to an earthquake of Richter scale magnitude 11.4–11.5) would have been released. In both cases, an impact-induced tsunamigenic earthquake could have caused the collapse of the local continental margin of the westernmost Massif Central and submarine landslides in the Aquitaine Basin and Biscay Rift as possible supporting mechanisms, maybe similar to the Eocene marine Montagnais impact scenario on the Scotian shelf of Eastern Canada (Jansa 1993). Comparable to tsunamis caused by endogenic earthquakes, an impact tsunami wave is thought to contain kinetic and gravitational energy in the range of about 7–9% of the total impact energy (Ahrens and O’Keefe 1983; Dypvik and Jansa 2003). According to Jansa (1993), a marine impact producing an approximately 50 km crater would result in a 200 m high wave within a radius of 300 km and still a 40 m wave within a 3000 km radius. The height of the tsunami wave is greatly amplified when reaching the shallow sea and coastal areas, especially when the coastal geometry (for example, V-shaped bays, channels, or fjords, etc.) supports effects of tsunami wave focusing and resonance (Satake 2007), as might have been the case on the Rhaetian British Isles.