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

Event- and biostratigraphic evidence for two independent Ries and Steinheim asteroid impacts in the Middle Miocene

For decades, the Nördlinger Ries and Steinheim Basin in southern Germany have been regarded as a textbook example of a terrestrial impact crater doublet, although the oldest crater lake deposits in both craters suggest a biostratigraphic age difference of ~ 0.5 to 1 Myr. We previously presented stratigraphic arguments that challenged the double impact scenario and favoured a model of two temporally independent impact events in the Mid-Miocene. We here present, for the first time, four localities within a distance of ~ 50-100 km from the Ries and ~ 50-70 km from the Steinheim crater that expose two independent seismite horizons, together unique within the Upper Freshwater Molasse of the North Alpine Foreland Basin, each one featuring impressive water escape structures. The seismite horizons are separated by ~ 10 to 15 m of undisturbed Molasse deposits and, biostratigraphically, by an entire European Land Mammal Zone, thus providing evidence for two independent major seismic events within a time span of ~ 0.5-1 Myr. Both the lower and the upper seismite horizons can be correlated litho- and biostratigraphically with the basal crater lake sediments at the Ries and Steinheim craters, respectively, deposited immediately after the impacts. From a biostratigraphic point of view, the impact event that formed the Steinheim Basin probably occured around 14 Ma, some 0.8 Myr after the ~ 14.81 Ma Ries impact event.

Sand spikes pinpoint powerful palaeoseismicity

Sand spikes, pin-shaped, carbonate-cemented sandstone bodies of variable size widely interpreted as sedimentary concretions, have been enigmatic for nearly two centuries. We here present a high-energy mechanism for their formation. Two classic sand spike occurrences are found in the North Alpine Foreland Basin of Central Europe and at Mount Signal in southern California, USA. A distinct seismite horizon in Mid-Miocene Molasse sediments of southern Germany, genetically linked with the Ries impact event, exhibits dewatering structures and contains numerous sand spikes with tails systematically orientated away from the Ries crater. Sand spikes at Mount Signal, strikingly similar in shape to those found in Germany, have tails that point away from the nearby San Andreas Fault. Based on their structural and stratigraphic context, we interpret sand spikes as a new type of seismite and a promising tool to identify strong impact-induced or tectonic palaeo-earthquakes and their source regions in the geologic record.

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

Impact ejecta formation and emplacement is of great importance when it comes to understanding the process of impact cratering and consequences of impact events in general. Here we present a multidisciplinary investigation of a distal impact ejecta layer, the Blockhorizont, that occurs near Bernhardzell in eastern Switzerland. We provide unambiguous evidence that this layer is impact-related by confirming the presence of shocked quartz grains exhibiting multiple sets of planar deformation features. Average shock pressures recorded by the quartz grains are ~ 19 GPa for the investigated sample. U–Pb dating of zircon grains from bentonites in close stratigraphic context allows us to constrain the depositional age of the Blockhorizont to ~ 14.8 Ma. This age, in combination with geochemical and paleontological analysis of ejecta particles, is consistent with deposition of this material as distal impact ejecta from the Ries impact structure, located ~ 180 km away, in Germany. Our observations are important for constraining models of impact ejecta emplacement as ballistically and non-ballistically transported fragments, derived from vastly different depths in the pre-impact target, occur together within the ejecta layer. These observations make the Ries ejecta one of the most completely preserved ejecta deposit on Earth for an impact structure of that size.