Sediment deformation structures in the Rhaetian of Luxembourg

The critical timespan during the Rhaetian (Norian to Hettangian), and in particular the Triassic–Jurassic transition, is known for the catastrophic end-Triassic mass extinction event (201.6 Ma). This occurred in the context of the breakup of the supercontinent Pangaea, and was accompanied by strong earthquakes and widespread volcanism.

In Luxembourg, the Rhaetian is exceptionally well preserved as a complete stratigraphic sequence in several drill cores (Elvange, Geyershaff, Grouft, and Heedhaff) and has been studied intensively in detail by the authors. It is subdivided in Luxembourg into a lower part, the Mortinsart Formation (Grès de Mortinsart), and an upper part, the Levallois Formation (Argiles de Levallois). The Mortinsart Formation is built up by alternating greyish-green sand-, and black claystones with rare thin black conglomerates and coaly horizons. They are overlain by the Levallois Formation, a very uniform sequence of reddish-brown claystones with thin silt streaks. Furthermore, the Levallois Formation shows numerous horizons with microfold structures in Luxembourg.

These soft-sediment deformation structures (SSDS) are usually interpreted as seismites because they are earthquake-induced and can be observed in the end-Triassic mass extinction interval across Europe. The origin of these is the intense seismic activity, linked to the formation of the Central Atlantic magmatic province (CAMP) caused by the breakup of Pangea.

The Levallois Formation is separated from the underlying Mortinsart Formation by an extremely chaotic, intensively deformed and completely unsorted horizon, which shows  flame structures, and vertebrate remains are enriched, including dinosaur bones. The regional distribution and geochemical fingerprints indicate that this could be a tsunamite generated by a potential asteroid impact. However, other causes may also have led to the formation of this horizon. Actually, detailed studies are ongoing.