The Monotis-Dactylioceras Bed in the Posidonienschiefer Formation (Toarcian, southern Germany): condensed section, tempestite, or tsunami-generated deposit?

Abstract

The Monotis-Dactylioceras Bed in the Toarcian Posidonienschiefer Formation is a conspicuous shell accumulation, which was used as marble in the late eighteenth century of Bavaria. The shell bed is characterized by left valves of the bivalve Meleagrinella substriata (von Munster) and local accumulations of the ammonite Dactylioceras athleticum (Simpson), and has previously been considered to reflect a regression or a storm event. Indeed, three different interpretations of the bed appear possible: (1) condensed section, (2) tempestite, or (3) tsunami deposit. New observations, specifically swaley cross-stratification and sole marks, point to an event-like formation of the bed. While most of the observations are consistent with the formation by a storm event, the singular occurrence within the 4.4 Ma lasting Lower Toarcian, which spanned about 4.4 Ma, and the inverse grading of the shells point to a possible tsunami origin. The latter hypothesis is supported by contemporaneous seismites in the western Tethyan realm, and micrite injections in strata below the Monotis-Dactylioceras bed, which may reflect quake-induced sediment liquefaction.