New discovery of two seismite horizons challenges the Ries–Steinheim double-impact theory

Abstract

The Nördlinger Ries and the Steinheim Basin are widely perceived as a Middle Miocene impact crater doublet. We discovered two independent earthquake-produced seismite horizons in North Alpine Foreland Basin deposits potentially related to both impacts. The older seismite horizon, demonstrated to be associated with the Ries impact, is overlain by distal impact ejecta in situ, forming a unique continental seismite-ejecta couplet within a distance of up to 180 km from the crater. The younger seismite unit, also produced by a major palaeo-earthquake, comprises clastic dikes that cut through the Ries seismite-ejecta couplet. The clastic dikes may have formed in response to the Steinheim impact, some kyr after the Ries impact, in line with paleontologic results that indicate a time gap of about 0.5 Myr between the Ries and Steinheim events. This interpretation suggests the Ries and Steinheim impacts represent two temporally separate events in Southern Germany that, thus, witnessed a double disaster in the Middle Miocene. The magnitude–distance relationship of seismite formation during large earthquakes suggests the seismic and destructive potential of impact-induced earthquakes may be underestimated.

The Chöpfi pinnacles near Winterthur, Switzerland: long-distance effects of the Ries impact-earthquake?

“…1B). As seen elsewhere in the Molasse basin (Buchner et al 2020), clay-rich subsurface layers (aquitards) can locally increase the amount of fluids within overlying sands, thereby supporting the formation of soft-sediment seismites on an outcrop scale during an earthquake. The rare exposure of fossil sand diapirs near Winterthur may, therefore, indicate water saturation of the local substrate and its interaction with strong seismic waves.…”

“…The mid-Miocene Ries impact in southern Germany is Central Europe's biggest cosmic disaster in Phanerozoic geologic history. While earlier theories posited the Ries impact was contemporaneous with the smaller nearby Steinheim event in a double-impact scenario, it was recently shown that the Ries, with a Langhian age of 14.81 Ma (Schmieder et al 2018), predates Steinheim (Serravallian) by a few hundred thousand years (Buchner et al 2020). In addition to the distribution of impact ejecta, the Ries impact triggered a powerful (M W ~ 8.5) earthquake with far-reaching environmental effects (Buchner et al 2020).…”

“…Geologic traces of the Ries event can be found in sediments of the Upper Freshwater Molasse ("Obere Süßwassermolasse", OSM) within the North Alpine Foreland Basin of southern Germany and northern Switzerland, where coarse-grained distal Ries ejecta form a mid-Miocene marker bed (e.g., Sach 1997;Hofmann 2008). At several localities across the basin, Ries ejecta overlie an at least ~ 3-5 m-thick seismite unit characterized by sedimentary features typical of soft-sediment deformation (Buchner et al 2020). The most distal effects of the Ries impact in the foreland basin, evidence of ballistic ejecta deposition and seismic shaking, are found within a maximum radial distance of ~ 180 km from the crater center (e.g., Buchner et al 2020).…”

“…At several localities across the basin, Ries ejecta overlie an at least ~ 3-5 m-thick seismite unit characterized by sedimentary features typical of soft-sediment deformation (Buchner et al 2020). The most distal effects of the Ries impact in the foreland basin, evidence of ballistic ejecta deposition and seismic shaking, are found within a maximum radial distance of ~ 180 km from the crater center (e.g., Buchner et al 2020).…”

Shocked quartz in distal ejecta from the Ries impact event (Germany) found at ~ 180 km distance, near Bernhardzell, eastern Switzerland

“…The structure formed at 14.808 ± 0.038 Ma 17 . The nearby ~ 3.8 km in diameter Steinheim impact structure 19 is considered to have formed simultaneously, although this has been challenged in a recent publication 20 . The target bedrock is composed of sedimentary rocks of Triassic to Cenozoic age underlain by crystalline basement (Ref.…”