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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.

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.

Spectacular sedimentary structures recently found in the Molasse Basin (Oligocene–Miocene) in southern Germany were produced by soft‐sediment deformation under highly unusual conditions. These large, apparently wedge‐like structures –‘loading fractures’– cut down into beds of marl and are filled with coarse sand and intraclasts of shale. Wrapping the sides of the structures is a thin, continuous bed of layered dark claystone – the ‘DCB’. The upper and lower layers of this bed are an organic‐rich clay; the middle layer is a laminated quartzite. The precursor of the DCB was a lacustrine gyttja rich in diatom frustules. It was supersaturated in silica as it was buried. Subsequent diffusion of oxygen into this gyttja at a burial depth of only a few metres resulted in the formation of Liesegang laminae of quartz. These laminae grew and amalgamated, forming the layer of laminated quartzite. The sediments overlying the DCB were eventually removed by erosion, probably in a high‐energy marine environment. This erosion cut down to the DCB but was unable to penetrate it. The DCB remained exposed on the sea floor until a sudden depositional event occurred – the deposition of a 2·5 metre thick bed of coarse sand with shale intraclasts. Although the DCB had been able to resist the submarine erosion, it could not support the load of this new bed. The quartzite layer in it therefore fractured, transferring that load down onto the underlying, still‐unconsolidated marl. The intraclast‐rich sands were forced down into this marl, carrying ahead of them the partly broken remains of the DCB.

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